ndk: <sys/cdefs.h> minor update

This patch fixes some formatting issues in <sys/cdefs.h>:

- change '#if __STDC_VERSION__ > xxx' into '#if defined(__STDC_VERSION__) && __STDC_VERSION > xxx'
- add __LIBC_HIDDEN__ definition (not technically required, but makes comparing the headers with upstream easier)

ndk/platforms/android-3/include/sys/cdefs_elf.h

@@ -95,6 +95,10 @@
 	__asm__(".section _sec\n\t.asciz _str\n\t.previous")
 #endif
 
+/* GCC visibility helper macro */
+#define __LIBC_HIDDEN__							\
+	__attribute__ ((visibility ("hidden")))
+
 #define	__IDSTRING(_n,_s)		__SECTIONSTRING(.ident,_s)
 
 #define	__RCSID(_s)			__IDSTRING(rcsid,_s)