libc: minimal: add prototype of rand()

Add prototype of rand() that can be defined in tests.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

lib/libc/minimal/include/stdlib.h

@@ -24,6 +24,8 @@ void *calloc(size_t nmemb, size_t size);
 void *realloc(void *ptr, size_t size);
 void *reallocarray(void *ptr, size_t nmemb, size_t size);
 
+int rand(void);
+
 #define abs(x) ((x) < 0 ? -(x) : (x))
 
 #ifdef __cplusplus