Cast function pointers without prototype before calling them (#2842)

clang complained that this is deprecated in all versions of standard C,
and unsupported in C2x.

src/execute.c

@@ -921,11 +921,11 @@ jv jq_next(jq_state *jq) {
       }
       struct cfunction* function = &frame_current(jq)->bc->globals->cfunctions[*pc++];
       switch (function->nargs) {
-      case 1: top = function->fptr(jq, in[0]); break;
-      case 2: top = function->fptr(jq, in[0], in[1]); break;
-      case 3: top = function->fptr(jq, in[0], in[1], in[2]); break;
-      case 4: top = function->fptr(jq, in[0], in[1], in[2], in[3]); break;
-      case 5: top = function->fptr(jq, in[0], in[1], in[2], in[3], in[4]); break;
+      case 1: top = ((jv (*)(jq_state *, jv))function->fptr)(jq, in[0]); break;
+      case 2: top = ((jv (*)(jq_state *, jv, jv))function->fptr)(jq, in[0], in[1]); break;
+      case 3: top = ((jv (*)(jq_state *, jv, jv, jv))function->fptr)(jq, in[0], in[1], in[2]); break;
+      case 4: top = ((jv (*)(jq_state *, jv, jv, jv, jv))function->fptr)(jq, in[0], in[1], in[2], in[3]); break;
+      case 5: top = ((jv (*)(jq_state *, jv, jv, jv, jv, jv))function->fptr)(jq, in[0], in[1], in[2], in[3], in[4]); break;
       // FIXME: a) up to 7 arguments (input + 6), b) should assert
       // because the compiler should not generate this error.
       default: return jv_invalid_with_msg(jv_string("Function takes too many arguments"));