value_string: Switch val from unsigned to int

There's nothing really preventing a user from user negative values.

Otherwise if we keep it like this then g++ is not happy when passing eg.
{ -2, "foobar" } when initializing a value_string array.

Change-Id: I754fa7e054cb89801ef82edc82199dcfbe59c6ab

include/osmocom/core/utils.h

@@ -40,7 +40,7 @@
 
 /*! A mapping between human-readable string and numeric value */
 struct value_string {
-	unsigned int value;	/*!< numeric value */
+	int value;		/*!< numeric value */
 	const char *str;	/*!< human-readable string */
 };