lib/pager: fix compiler warnings

lib/pager: fix compiler warnings

pager.c: In function ‘start_command’:
pager.c:82:7: warning: unused variable ‘err’ [-Wunused-variable]
pager.c:49:25: warning: unused variable ‘fderr’ [-Wunused-variable]
pager.c:49:15: warning: unused variable ‘fdout’ [-Wunused-variable]
pager.c:48:25: warning: unused variable ‘need_err’ [-Wunused-variable]
pager.c:48:15: warning: unused variable ‘need_out’ [-Wunused-variable]

Signed-off-by: Karel Zak <kzak@redhat.com>