utils.h: #include <stdio.h> as we use fprintf()

Change-Id: I911c7f4bcadde414ce0b384e13a3a9a4a953e2fb
diff --git a/include/osmocom/core/utils.h b/include/osmocom/core/utils.h
index 41bbc27..d9c3097 100644
--- a/include/osmocom/core/utils.h
+++ b/include/osmocom/core/utils.h
@@ -21,6 +21,7 @@
 #define OSMO_VALUE_STRING(x) { x, OSMO_STRINGIFY(x) }
 
 #include <stdint.h>
+#include <stdio.h>
 
 /*! \brief A mapping between human-readable string and numeric value */
 struct value_string {