swr: automake: add missing -I flag

swr: automake: add missing -I flag

When building from a release tarball (where the generated/built files
are in srcdir) in an OOT fashion we need to have both builddir and
srcdir in the includes list.

Otherwise we'll error out, as the file (header gen_knobs.h in this case)
won't be in the location where we are looking.

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Cc: Tim Rowley <timothy.o.rowley@intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>