cairo: improve the Blackfin build fix

Patch cairo-001-blackfin-build-fix.patch was added in commit
d372d0b6 ('cairo: add patch to fix
Blackfin build failure') to take into account the fact that Blackfin
has a non-empty USER_LABEL_PREFIX: C symbol 'foo' corresponds to
assembly symbol '_foo', contrary to most other architectures where
they are the same.

However, the patch was only fixing the relevant macros in
src/cairo-compiler-private.h and did not take into account the
duplicated version of these macros in
util/cairo-script/cairo-script-private.h. This commit fixes that.

This commit is fixing a build failure on Blackfin, for which there was
no recorded autobuilder result because this problem was hidden by the
problem fixed by the previous commit ('cairo: fix build on Blackfin in
test/cairo-test-runner.c').

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>