e2fsck: Don't use e2fsck_global_ctx unless HAVE_SIGNAL_H

e2fsck: Don't use e2fsck_global_ctx unless HAVE_SIGNAL_H

Program received signal SIGSEGV, Segmentation fault.
0x56577382 in e2fsck_set_bitmap_type (fs=0x5658c2b8, default_type=2, profile_name=0x56580e12 "default", old_type=0x0) at external/e2fsprogs/e2fsck/util.c:797
797             profile_get_uint(e2fsck_global_ctx->profile, "bitmaps",

e2fsck_global_ctx is only ever initialized in unix.c PRS()
if HAVE_SIGNAL_H is defined.

Change-Id: Ic93fb8d9114054f7bf8512c622b2fb9b2ae60584
Signed-off-by: JP Abgrall <jpa@google.com>