easy_lock.h: remove use of the deprecated ATOMIC_VAR_INIT macro

clang 14 warns about its use. It is being deprecated by the working
group for the programming language C: "The macro ATOMIC_VAR_INIT is
basically useless for the purpose for which it was designed"

Ref: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2886.htm

Reported-by: Tatsuhiro Tsujikawa
Fixes curl#9041
Closes curl#9042

lib/easy_lock.h

@@ -38,7 +38,7 @@
 #include <stdatomic.h>
 
 #define curl_simple_lock atomic_bool
-#define CURL_SIMPLE_LOCK_INIT ATOMIC_VAR_INIT(false)
+#define CURL_SIMPLE_LOCK_INIT false
 
 static inline void curl_simple_lock_lock(curl_simple_lock *lock)
 {