pcre2posix.h missing from vendored copy of pcre2

It seems like pcre2posix.h plays double duty as both a source file within the pcre2 project and also as an interface header included in system header distributions of pcre2. It looks like the vendored copy of pcre2 is missing this file, which causes it to fail to compile on systems where the compiler doesn't have a system-installed copy of pcre2 and its api headers.

When I ran the build on my system, pcre2posix.c failed to find pcre2posix.h, because the build runs through msvc because I'm on windows, and I only have pcre2 installed as a system-accessible library inside of msys2.

Copy pasting pcre2posix.h from the msys2 distributions of pcre2 into the vendored copy of pcre2 seemed to fix it and let it build, but this is a hack and probably wrong because it'll go out of sync eventually as pcre2 changes.