Move elf definitions to a header file

They're going to be used by a unit test in the future.

Bug: 200136875
Change-Id: Id89370ad46af33dc42c8c3f541be2d7e0adf8565
diff --git a/BUILD b/BUILD
index 3124f4b..310f730 100644
--- a/BUILD
+++ b/BUILD
@@ -791,6 +791,7 @@
         "src/profiling/symbolizer/breakpad_parser.h",
         "src/profiling/symbolizer/breakpad_symbolizer.cc",
         "src/profiling/symbolizer/breakpad_symbolizer.h",
+        "src/profiling/symbolizer/elf.h",
         "src/profiling/symbolizer/filesystem.h",
         "src/profiling/symbolizer/filesystem_posix.cc",
         "src/profiling/symbolizer/filesystem_windows.cc",