util/string-util: export skip_space

It's only few lines, but we already define the function, so make it
usable elsewhere

util/string-util.c

@@ -141,7 +141,7 @@ make_boolean_term (void *ctx, const char *prefix, const char *term,
     return 0;
 }
 
-static const char*
+const char*
 skip_space (const char *str)
 {
     while (*str && isspace ((unsigned char) *str))

util/string-util.h

@@ -77,6 +77,8 @@ unsigned int strcase_hash (const void *ptr);
 
 void strip_trailing (char *str, char ch);
 
+const char* skip_space (const char *str);
+
 #ifdef __cplusplus
 }
 #endif