pregcomp2.c: Avoid using incompatible pointer type

[chrstphrchvz]

See https://github.com/eserte/perl-tk/issues/98#issuecomment-1948125587: Newer compilers may have -Werror=incompatible-pointer-types by default. Since pregcomp2.c does not dereference the result of pregcomp2(), the result can be assigned to void * instead. There is already the regexp511.c configuration test to decide whether to use REGEXP * instead of regexp *, but that test is run after pregcomp2.c.