wcsmbs/bits/wchar2{, -decl}.h: Clearly separate declaration from definitions

This will enable __REDIRECT_FORTIFY* macros to be used when _FORTIFY_SOURCE
is set.

Routine declarations that were in bits/wchar2.h are moved into the
bits/wchar2-decl.h file.
The file is now included into include/wchar.h irrespectively from
fortification.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>