Move epoxy_headers definition into include/epoxy

In order to properly depend on headers, both generated and provided, in
a separate directory, we need to refer to them using their path.

Generated headers already have their full path; for provided ones, we
can simply use the `files()` directive.

This change should allow using libepoxy as a Meson subproject.

Fixes: #115

include/epoxy/meson.build

@@ -39,4 +39,6 @@ foreach g: generated_headers
   headers += [ header ]
 endforeach
 
+epoxy_headers = files(headers) + gen_headers
+
 install_headers(headers, subdir: 'epoxy')

src/meson.build

@@ -46,10 +46,6 @@ foreach g: generated_sources
 endforeach
 
 epoxy_sources = sources + gen_sources
-epoxy_headers = gen_headers
-foreach h: headers
-  epoxy_headers += join_paths(meson.source_root(), 'include/epoxy/@0@'.format(h))
-endforeach
 
 common_ldflags = []