build: fix redundant redeclaration warnings

build: fix redundant redeclaration warnings

env.c:24:15: warning: redundant redeclaration of 'environ' [-Wredundant-decls]
su.c:81:15: warning: redundant redeclaration of 'environ' [-Wredundant-decls]

fstab.c:581:14: warning: redundant redeclaration of 'strsignal' [-Wredundant-decls]

kill.h:1:13: note: previous declaration of 'get_pids' was here
kill.c:152:13: warning: redundant redeclaration of 'get_pids' [-Wredundant-decls]

kill.c:142:5: warning: redundant redeclaration of 'main' [-Wredundant-decls]
getopt.c:89:5: warning: redundant redeclaration of 'main' [-Wredundant-decls]

agetty.c:536:15: warning: redundant redeclaration of 'optarg' [-Wredundant-decls]
agetty.c:537:13: warning: redundant redeclaration of 'optind' [-Wredundant-decls]
script.c:161:13: warning: redundant redeclaration of 'optind' [-Wredundant-decls]
wall.c:96:13: warning: redundant redeclaration of 'optind' [-Wredundant-decls]

libmount.h:362:26: note: previous declaration of 'mnt_update_get_fs' was here
libmount.h:454:26: note: previous declaration of 'mnt_context_get_fs' was here
mountP.h:383:26: warning: redundant redeclaration of 'mnt_context_get_fs' [-Wredundant-decls]
mountP.h:398:26: warning: redundant redeclaration of 'mnt_update_get_fs' [-Wredundant-decls]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>