Mark libpcre2 as double_loadable

libpcre2 is a VNDK lib and also used by LLNDK(libmediandk) which means
this lib can be double-loaded.
(deps: libmediandk -> libmedia_jni -> libandroid_runtime -> libselinux
-> libpcre2)

Bug: 121280180
Test: m -j
Change-Id: I2a9750dcee4f99119ce9af94a2034a45278a2ea3