scons: do not include headers from the sources lists

The SCons documentation is not explicit on the topic yet building mesa
with SCons and MSVC is known to have problems when headers are listed.
So be safe just drop them for now.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82534
Tested-by: Vinson Lee <vlee@freedesktop.org>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>

scons/custom.py

@@ -276,6 +276,9 @@ def parse_source_list(env, filename, names=None):
                     # Prefer relative source paths, as absolute files tend to
                     # cause duplicate actions.
                     f = f[len(cur_srcdir + '/'):]
+                # do not include any headers
+                if f.endswith('.h'):
+                    continue
                 srcs.append(f)
 
         src_lists[sym] = srcs