tree-wide: drop "static inline" use in .c files

"static inline" makes sense in .h files. But in .c files it's useless
decoration, the compiler should just make its own decisions there, and
it can do that.

hence, replace all remaining uses of "static line" by a simple" static"
in all .c files (but keep them in .h files, where they make sense)

src/sysusers/sysusers.c

@@ -47,7 +47,7 @@ typedef enum ItemType {
         ADD_RANGE =  'r',
 } ItemType;
 
-static inline const char* item_type_to_string(ItemType t) {
+static const char* item_type_to_string(ItemType t) {
         switch (t) {
         case ADD_USER:
                 return "user";