Fixed include path for header file poll.h.

[ii8]

The correct include path for poll(2) is poll.h not sys/poll.h. sys/poll.h may not be available on some libcs or may issue a warning. In particular this fixes a warning on musl systems.

/usr/include/sys/poll.h:1:2: error: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Werror=cpp]
    1 | #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
      |  ^~~~~~~
cc1: all warnings being treated as errors
make[3]: *** [CMakeFiles/st-util.dir/build.make:76: CMakeFiles/st-util.dir/src/st-util/gdb-remote.c.o] Error 1
make[2]: *** [CMakeFiles/Makefile2:342: CMakeFiles/st-util.dir/all] Error 2
make[1]: *** [Makefile:156: all] Error 2
make: *** [Makefile:34: release] Error 2