libc: Fix SIZE_MAX on AArch32

SIZE_MAX was mistakenly redefined from UINT32_MAX to UINT64_MAX
on AArch32 when the arch-specific headers were merged.

This value is not currently used by upstream TF-A source code,
so no functionality should be affected.

Change-Id: I2acf7f8736423697c7377e8ed4b08843ced26e66
Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>

include/lib/libc/stdint.h

@@ -72,7 +72,7 @@
 #define PTRDIFF_MIN LONG_MIN
 #define PTRDIFF_MAX LONG_MAX
 
-#define SIZE_MAX UINT64_MAX
+#define SIZE_MAX ULONG_MAX
 
 #define INT8_C(x)  x
 #define INT16_C(x) x