Rename duplicate source files

This fixes an error with toolchains that don't include the extension in
the .o filename.

Change-Id: I1fe46d127cade90e6c3d330ee45937c2ea8665eb
diff --git a/pw_assert/BUILD.gn b/pw_assert/BUILD.gn
index 417e0b2..d8d1ad1 100644
--- a/pw_assert/BUILD.gn
+++ b/pw_assert/BUILD.gn
@@ -69,8 +69,8 @@
     pw_assert_BACKEND,
   ]
   sources = [
-    "assert_backend_compile_test.c",
     "assert_backend_compile_test.cc",
+    "assert_backend_compile_test_c.c",
   ]
 }