Make helper functions static.

Found by -Wmissing-prototypes. NFC.

llvm-svn: 231664
diff --git a/llvm/tools/llvm-cov/gcov.cpp b/llvm/tools/llvm-cov/gcov.cpp
index c0a48f8..4377a50 100644
--- a/llvm/tools/llvm-cov/gcov.cpp
+++ b/llvm/tools/llvm-cov/gcov.cpp
@@ -23,9 +23,10 @@
 #include <system_error>
 using namespace llvm;
 
-void reportCoverage(StringRef SourceFile, StringRef ObjectDir,
-                    const std::string &InputGCNO, const std::string &InputGCDA,
-                    bool DumpGCOV, const GCOVOptions &Options) {
+static void reportCoverage(StringRef SourceFile, StringRef ObjectDir,
+                           const std::string &InputGCNO,
+                           const std::string &InputGCDA, bool DumpGCOV,
+                           const GCOVOptions &Options) {
   SmallString<128> CoverageFileStem(ObjectDir);
   if (CoverageFileStem.empty()) {
     // If no directory was specified with -o, look next to the source file.