string.h: only guard snprintf() with __ROMCC__

There's no need to keep the snprintf() declaration hidden
for early stages. romcc is the entity that has issues. Therefore,
be explicit about when to guard snprintf().

BUG=chromium:663243

Change-Id: Ib4d0879e52c3f73c6ca61ab75f672f0003fca71f
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/17289
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/include/string.h

@@ -15,7 +15,7 @@ void *memmove(void *dest, const void *src, size_t n);
 void *memset(void *s, int c, size_t n);
 int memcmp(const void *s1, const void *s2, size_t n);
 void *memchr(const void *s, int c, size_t n);
-#if !defined(__PRE_RAM__)
+#if !defined(__ROMCC__)
 int snprintf(char * buf, size_t size, const char *fmt, ...);
 #endif