Explicitly disable -Werror for builds
The use of %autosetup -S git_am clashes with spice-compile-warnings.m4
handling of -Werror.
It automatically enables -Werror for builds from git, which are detected
through the presence of a .git directory is $srcdir. %autosetup will
create such a dir, so -Werror will be enabled, which is unwanted and a
change of behaviour from previous builds.