webclient.h: Avoid relying on indirect inclusion

Include stdint.h for uint16_t explicitly.

This was necessary when I was trying to build this natively
on Ubuntu.  It seems some other headers happen to pull
the uint16_t definition by luck on NuttX and macOS.

include/netutils/webclient.h

@@ -49,6 +49,7 @@
 #include <nuttx/config.h>
 
 #include <stdbool.h>
+#include <stdint.h>
 #include <sys/types.h>
 
 /****************************************************************************