Addressed a warning in mk_gmtime() when compiler flag -Wextra is used

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2333 1084037a-409f-4a99-8160-1a4b3577d4c1
Fixed gm_sidereal() so it will not generate warning -Werror=long-long when compiled with -Wextra

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2334 1084037a-409f-4a99-8160-1a4b3577d4c1