Disable -Wimplicit-fallthrough for funcs.generated.h

This is code generated by gperf, so there's nothing we can do about it.
The code does have /*FALLTHROUGH*/ comments but clang doesn't recognize
them.

src/nvim/eval/funcs.c

@@ -32,6 +32,7 @@
 #include "nvim/indent.h"
 #include "nvim/indent_c.h"
 #include "nvim/lua/executor.h"
+#include "nvim/macros.h"
 #include "nvim/mark.h"
 #include "nvim/math.h"
 #include "nvim/memline.h"
@@ -86,8 +87,10 @@ KHASH_MAP_INIT_STR(functions, VimLFuncDef)
 #endif
 
 PRAGMA_DIAG_PUSH_IGNORE_MISSING_PROTOTYPES
+PRAGMA_DIAG_PUSH_IGNORE_IMPLICIT_FALLTHROUGH
 #include "funcs.generated.h"
 PRAGMA_DIAG_POP
+PRAGMA_DIAG_POP
 #endif