Makefile.am: fix "make dist"

Several files were renamed in 15352c63c239af1aad43f551123facf49448efe7
without also fixing their entries in Makefile.am.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Makefile.am: clean out EXTRA_DIST

Several header files were listed in EXTRA_DIST.  Moved them all to
their corresponding _SOURCES macros.  This keeps core/provider header
files in their own distinct sections (vs. glumping .h files from
different providers in EXTRA_DIST).  The sockets .h file was already
listed in its _SOURCES macro.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Merge pull request #235 from jsquyres/pr/fix-make-dist

2 fixes for "make dist"