secmem: Export symbols and the interface.
* src/Makefile.am (libgpg_error_la_SOURCES): Add secmem.c and secmem.h. * src/gpg-error.def.in: Add secmem symbols. * src/gpg-error.vers: Likewise. * src/gpg-error.h.in: Add secmem functions and macros. * src/secmem.h: Move internal prototypes to ... * src/gpgrt-int.h: ... here. * src/visibility.c: Add external functions. * src/visibility.h: Declare secmem functions.