versioner: ignore non-header files.

Don't try to compile non-headers (vim swap files, etc.) that happen to
be in the header directory.

Bug: none
Test: manually created a file in libc/include
Change-Id: I2c1647ec7174dc617898dc2f07c9a80a08b69c72
diff --git a/tools/versioner/src/versioner.cpp b/tools/versioner/src/versioner.cpp
index 86349e1..0cb7bd0 100644
--- a/tools/versioner/src/versioner.cpp
+++ b/tools/versioner/src/versioner.cpp
@@ -56,7 +56,7 @@
 
 static CompilationRequirements collectRequirements(const Arch& arch, const std::string& header_dir,
                                                    const std::string& dependency_dir) {
-  std::vector<std::string> headers = collectFiles(header_dir);
+  std::vector<std::string> headers = collectHeaders(header_dir);
   std::vector<std::string> dependencies = { header_dir };
   if (!dependency_dir.empty()) {
     auto collect_children = [&dependencies, &dependency_dir](const std::string& dir_path) {