Resolve several missing declarations warnings

src/gflags_completions.cc

@@ -47,6 +47,8 @@
 //     5b) Trim most flag's descriptions to fit on a single terminal line
 
 
+#include "gflags_completions.h"
+
 #include "config.h"
 
 #include <stdio.h>

test/gflags_declare_flags.cc

@@ -3,6 +3,7 @@
 
 DECLARE_string(message); // in gflags_delcare_test.cc
 
+void print_message();
 void print_message()
 {
   std::cout << FLAGS_message << std::endl;

test/gflags_unittest.cc

@@ -1485,7 +1485,7 @@ TEST(FlagsValidator, FlagSaver) {
 
 }  // unnamed namespace
 
-int main(int argc, char **argv) {
+static int main(int argc, char **argv) {
 
   // Run unit tests only if called without arguments, otherwise this program
   // is used by an "external" usage test