util: Make in_charset a static inline in util.h

With in_charset now reduced to a one-liner (plus asserts), make it a
static inline.

src/shared/util.c

@@ -920,12 +920,6 @@ char *delete_chars(char *s, const char *bad) {
         return s;
 }
 
-bool in_charset(const char *s, const char* charset) {
-        assert(s);
-        assert(charset);
-        return s[strspn(s, charset)] == '\0';
-}
-
 char *file_in_same_dir(const char *path, const char *filename) {
         char *e, *r;
         size_t k;

src/shared/util.h

@@ -531,7 +531,11 @@ char *strjoin(const char *x, ...) _sentinel_;
 
 bool is_main_thread(void);
 
-bool in_charset(const char *s, const char* charset) _pure_;
+static inline bool _pure_ in_charset(const char *s, const char* charset) {
+        assert(s);
+        assert(charset);
+        return s[strspn(s, charset)] == '\0';
+}
 
 int block_get_whole_disk(dev_t d, dev_t *ret);