docs: libnm: add type headers to scan list

nm-core-types.h and nm-types.h contain the actual definition of types
and gtk-doc won't generate a "Implemented interfaces" section if they
are not included.

https://bugzilla.gnome.org/show_bug.cgi?id=765983
(cherry picked from commit a51a43fc84)

docs/libnm/Makefile.am

@@ -38,7 +38,6 @@ IGNORE_HFILES= \
 	crypto.h \
 	nm-dbus-helpers.h \
 	nm-core-internal.h \
-	nm-core-types.h \
 	nm-core-types-internal.h \
 	nm-device-private.h \
 	nm-dhcp4-config.h \
@@ -52,7 +51,6 @@ IGNORE_HFILES= \
 	nm-remote-connection-private.h \
 	nm-remote-settings.h \
 	nm-setting-private.h \
-	nm-types.h \
 	nm-utils-private.h \
 	nm-core-tests-enum-types.h \
 	nm-keyfile-internal.h \