util: fix build with CFLAGS="-g"

The build process fails with debug symbols enabled. It appears that
`CFLAGS=-g` influences the compiler's inlining decisions, leading to a
situation where the `is_str_empty` function is not inlined, and the
linker is searching for an out-of-line definition that isn’t present.
The most straightforward fix is to declare this function
`static inline`.

Signed-off-by: Wolfgang Frisch <[email protected]>

util.h

@@ -33,7 +33,7 @@ char* trim_space(char *str);
 int get_abbreviated(uint64_t num, int max_digs, char *res);
 int contains(const char *prfx, const char *str);
 
-inline int is_str_empty(char *str)
+static inline int is_str_empty(char *str)
 {
 	if (str && str[0] == '\0')
 		return 1;