fix: compiler warnings from clang 15 (#20321)

Add -Wno-strict-prototypes flag to external dependencies to suppress
cjson warnings. These needs to be fixed upstream first.

src/nvim/input.c

@@ -83,7 +83,6 @@ int get_keystroke(MultiQueue *events)
   int len = 0;
   int n;
   int save_mapped_ctrl_c = mapped_ctrl_c;
-  int waited = 0;
 
   mapped_ctrl_c = 0;        // mappings are not used here
   for (;;) {
@@ -110,10 +109,8 @@ int get_keystroke(MultiQueue *events)
       // Replace zero and K_SPECIAL by a special key code.
       n = fix_input_buffer(buf + len, n);
       len += n;
-      waited = 0;
-    } else if (len > 0) {
-      waited++;             // keep track of the waiting time
     }
+
     if (n > 0) {  // found a termcode: adjust length
       len = n;
     }