build: -Wmissing-prototypes

ref #343

Though I don't see a strong benefit, it isn't too much of a burden, and
maybe avoids confusion in some cases.

src/nvim/eval.c

@@ -5958,6 +5958,10 @@ static int get_env_tv(char_u **arg, typval_T *rettv, int evaluate)
 #pragma function (floor)
 #endif
 
+// silence -Wmissing-prototypes for funcs.generated.h
+const VimLFuncDef *find_internal_func_gperf(register const char *str,
+                                            register size_t len);
+
 # include "funcs.generated.h"
 #endif