tree-wide: use __ prefixed gcc attributes (#10843)

As suggest here:

https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#Attribute-Syntax

    "You may optionally specify attribute names with ‘__’ preceding and
    following the name. This allows you to use them in header files without
    being concerned about a possible macro of the same name. For example,
    you may use the attribute name __noreturn__ instead of noreturn. "

src/basic/util.h

@@ -172,7 +172,8 @@ static inline void _reset_errno_(int *saved_errno) {
         errno = *saved_errno;
 }
 
-#define PROTECT_ERRNO _cleanup_(_reset_errno_) __attribute__((unused)) int _saved_errno_ = errno
+#define PROTECT_ERRNO                                                   \
+        _cleanup_(_reset_errno_) __attribute__((__unused__)) int _saved_errno_ = errno
 
 static inline int negative_errno(void) {
         /* This helper should be used to shut up gcc if you know 'errno' is