audio/libmatemixer: drop gtk-doc API documentation

The release tarball ships pre-built gtk-doc HTML files that get
installed unconditionally despite --disable-gtk-doc being set in
CONFIGURE_ARGS. Patch Makefile.in to remove the docs directory
from SUBDIRS as API documentation takes unnecessary disk space
on end user systems.

audio/libmatemixer/files/patch-Makefile.in

@@ -0,0 +1,9 @@
+--- Makefile.in.orig
++++ Makefile.in
+@@ -389,7 +389,6 @@ SUBDIRS = \
+ 	po                              \
+ 	libmatemixer                    \
+ 	backends                        \
+ 	data                            \
+-	docs                            \
+ 	examples