lib_package.bbclass: Use += to add to PACKAGE_BEFORE_PN

lib_package.bbclass: Use += to add to PACKAGE_BEFORE_PN

The bash-completion bbclass recently started to use PACKAGE_BEFORE_PN to
add the ${PN}-bash-completion package. This meant that a recipe, e.g.,
proj in meta-oe, that inherits both bash-completion and lib_package
would fail since lib_package set PACKAGE_BEFORE_PN rather than added to
it.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>