secmem: Exposed function should have consistent name.
* src/gpg-error.def.in, src/gpg-error.h.in, src/gpg-error.vers: Rename to gpgrt_secmem_is_secure (was: gpgrt_private_is_secure). * src/gpgrt-int.h, src/secmem.c: Reneme for _gpgrt_private_is_secure. * src/visibility.c, src/visibility.h: Follow the rename.