tools/gpio: Fix build error with musl libc

The GPIO tools build fails when using a buildroot toolchain that uses musl
as it's C library:

arm-broomstick-linux-musleabi-gcc -Wp,-MD,./.gpio-event-mon.o.d \
 -Wp,-MT,gpio-event-mon.o -O2 -Wall -g -D_GNU_SOURCE \
 -Iinclude -D"BUILD_STR(s)=#s" -c -o gpio-event-mon.o gpio-event-mon.c
gpio-event-mon.c:30:6: error: unknown type name ‘u_int32_t’; did you mean ‘uint32_t’?
      u_int32_t handleflags,
      ^~~~~~~~~
      uint32_t

The glibc headers installed on my laptop include sys/types.h in
unistd.h, but it appears that musl does not.

Fixes: 97f69747 ("tools/gpio: add the gpio-event-mon tool")
Cc: stable@vger.kernel.org
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>