tools/nxstyle: Whitelist Micro XRCE-DDS uxrCustomTransport prefix

[arjav1528]

Summary

Follow-up to bc3a259 (tools/nxstyle: Whitelist ROS 2 message type identifiers.). The Micro XRCE-DDS Client custom-transport API
expects user callbacks of the form

bool   open_cb (struct uxrCustomTransport *transport);
bool   close_cb(struct uxrCustomTransport *transport);
size_t write_cb(struct uxrCustomTransport *transport,
                const uint8_t *buf, size_t len, uint8_t *err);
size_t read_cb (struct uxrCustomTransport *transport,
                uint8_t *buf, size_t len, int timeout, uint8_t *err);

The uxrCustomTransport tag is fixed by the upstream public header
<uxr/client/profile/transport/custom/custom_transport.h> and cannot
be renamed without breaking the Micro XRCE-DDS C API.

Add the uxrCustom prefix to g_white_prefix so nxstyle stops
flagging every callback signature in the upcoming
apps/system/microros/transport backend with
error: Mixed case identifier found.

Impact

• tools/nxstyle only. No build, runtime, or hardware effect.
• Pure relaxation of the style checker for a single, enumerated
  identifier prefix. No existing in-tree code becomes invalid.

Dependencies

Companion to apache/nuttx-apps#3512 (system/microros: Add UDP and serial custom transports). That PR's CI nxstyle step otherwise
fails on every struct uxrCustomTransport * parameter in the new
transport backend (~20 errors across 4 files).

Testing

Local build of the patched tools/nxstyle and run against the four
transport files plus the publisher example in apache/nuttx-apps#3512:

$ gcc -O2 -o tools/nxstyle tools/nxstyle.c
$ for f in \
    apps/system/microros/transport/microros_transport.c \
    apps/system/microros/transport/microros_transport.h \
    apps/system/microros/transport/microros_transport_udp.c \
    apps/system/microros/transport/microros_transport_serial.c \
    apps/examples/microros_pub/microros_pub_main.c; do
    ./tools/nxstyle "$f"; done
(no output -> all clean)

Before this patch (same nxstyle build minus the new whitelist
entry):

microros_transport.h:58:32: error: Mixed case identifier found
microros_transport.h:59:33: error: Mixed case identifier found
microros_transport_udp.c:44:32: error: Mixed case identifier found
... (~20 errors total across the four files)