fix(libc): snprintf: include stdint.h

The snprintf code uses the uintptr_t type, which is defined in stdint.h.
We do not include this header explicitly, but get the definition
indirectly through some other header doing so.

However this breaks when snprintf is compiled in isolation (for instance
for unit-testing), so let's add this #include to make things right.

Change-Id: I1299767ee482f5cf1af30c4df2e8f7e596969b41
Signed-off-by: Andre Przywara <andre.przywara@arm.com>