lib: Add obvious definition for calloc

The calloc() function is useful in addition to malloc and friends, so
add the obvious definition.

Change-Id: I57a568e323344a97b35014b7b8bec16adc2fd720
Signed-off-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51949
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/stdlib.h

@@ -5,6 +5,7 @@
 
 void *memalign(size_t boundary, size_t size);
 void *malloc(size_t size);
+void *calloc(size_t nitems, size_t size);
 void free(void *ptr);
 
 #endif /* STDLIB_H */