avahi: fix libgtk3 build issue by patching Makefile.in instead of Makefile.am

Fixes:
http://autobuild.buildroot.net/results/f0b/f0b4f8c46eb1de6cf5ed081dacd248c66c591aa3/
http://autobuild.buildroot.net/results/dac/daca5d030b8ecc6d11901e2936fe2151d0930852/
http://autobuild.buildroot.net/results/05e/05e7fb9a888169ba7c4cb28abdb4febb5215354c/
http://autobuild.buildroot.net/results/743/743f30b1816b17a0a1397b7fd8b5a8571c4dde03/



And many others.

So we no longer need to autoreconf the package. Autoreconfing is non-trivial
as it uses m4 macros from glib, but we don't want to unconditionally pull in
glib.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>