Fix Alpine's build in the CI

PCRE must now be installed, since it's
not shipped with PHP by default anymore.
Fedora did it too, and was fixed in 3fe0d28.

.gitlab-ci.yml

@@ -29,7 +29,7 @@ testsuite:alpine:
   stage: testsuite
   script:
     - apk update
-    - apk add php7-dev php7-cgi php7-simplexml php7-xml make gcc musl-dev
+    - apk add php7-dev php7-cgi php7-simplexml php7-xml make gcc musl-dev pcre
     - make compile_debug
     - TEST_PHP_ARGS='-q' REPORT_EXIT_STATUS=1 make -C src test TESTS="tests/cookies_encryption tests/deny_writable tests/disable_function"
     - TEST_PHP_ARGS='-q' REPORT_EXIT_STATUS=1 make -C src test TESTS="tests/dump_request tests/eval_blacklist tests/global_strict"