item_cmpfunc.h includes pcre2.h, so with the bundled pcre2
it has to be built before anything that includes pcre2.h .
And item_cmpfunc.h is indirectly included everywhere,
also in many plugins.

Somehow Ninja and Makefiles generators can still deduce the
correct build dependencies, but Visual Studio generator cannot.

Two changes:
* move pcre2.h from item_cmpfunc.h to item_cmpfunc.cc
* create an explicit dependency on pcre2 for the server