Add this to Info.plist in /System/Library/Spotlight/RichText.mdimporter/Contents/ and Spotlight will search for source code files.
<string>public.c-header</string>
<string>public.c-plus-plus-header</string>
<string>public.c-source</string>
<string>public.objective-c-source</string>
public.c-plus-plus-source