gpio: add feature test macro for POLLRDNORM

The code needs _XOPEN_SOURCE for some poll() flags.

Fix build with uclibc:
src/gpio.c: In function gpio_poll_multiple»:
src/gpio.c:242:61: error: «POLLRDNORM» undeclared (first use in this function)
(POLLPRI | POLLERR) : (POLLIN | POLLRDNORM);
                                ^

closes #24.

Signed-off-by: Joris Offouga <offougajoris@gmail.com>
Signed-off-by: Vanya A. Sergeev <v@sergeev.io>

src/gpio.c

@@ -4,6 +4,8 @@
  * License: MIT
  */
 
+#define _XOPEN_SOURCE   /* for POLLRDNORM */
+
 #include <stddef.h>
 #include <stdbool.h>
 #include <stdarg.h>