console: Expose vsnprintf

It's a standard function.

Change-Id: I039cce2dfc4e168804eb7d12b76a29af712ac7a1
Signed-off-by: David Hendricks <dhendricks@fb.com>
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/23616
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/string.h

@@ -4,6 +4,10 @@
 #include <stddef.h>
 #include <stdlib.h>
 
+#if !defined(__ROMCC__)
+#include <console/vtxprintf.h>
+#endif
+
 /* Stringify a token */
 #ifndef STRINGIFY
 #define _STRINGIFY(x)	#x
@@ -17,6 +21,7 @@ int memcmp(const void *s1, const void *s2, size_t n);
 void *memchr(const void *s, int c, size_t n);
 #if !defined(__ROMCC__)
 int snprintf(char *buf, size_t size, const char *fmt, ...);
+int vsnprintf(char *buf, size_t size, const char *fmt, va_list args);
 #endif
 
 // simple string functions