[gflags] Use void (*)() for ValidateFnProto

Change ValidateFnProto from bool (*)() to void (*)(). In C++,
bool (*)() is a function taking zero arguments, which triggers
-Wcast-function-type-mismatch when casting to and from 2-argument
validator function pointers.

void (*)() is treated as a generic function pointer by Clang and GCC,
allowing opaque storage without compiler warnings.

Bug: 330769701
Change-Id: I38c4f710b83bcd98dcac147433f68602a33d8e70
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/gflags/+/1788820
Reviewed-by: Francois Rousseau <frousseau@google.com>

src/gflags.cc

diff --git a/src/gflags.cc b/src/gflags.cc
index 921b4f9..bbd6cc8 100644
--- a/src/gflags.cc
+++ b/src/gflags.cc
@@ -164,7 +164,7 @@
 // ValidateFn(uint64).  However, for easier storage, we strip off this
 // argument and then restore it when actually calling the function on
 // a flag value.
-typedef bool (*ValidateFnProto)();
+typedef void (*ValidateFnProto)();
 
 // Whether we should die when reporting an error.
 enum DieWhenReporting { DIE, DO_NOT_DIE };