pagemap: include falloc.h for FALLOC_FL_PUNCH_HOLE

Otherwise on some older environments like RHEL7 it will fail to build with:
  CC       pagemap.o
pagemap.c: In function ‘punch_hole’:
pagemap.c:54:40: error: ‘FALLOC_FL_PUNCH_HOLE’ undeclared (first use in this function)
    ret = fallocate(img_raw_fd(pr->pi), FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
pagemap.c:54:40: note: each undeclared identifier is reported only once for each function it appears in
pagemap.c:54:63: error: ‘FALLOC_FL_KEEP_SIZE’ undeclared (first use in this function)
    ret = fallocate(img_raw_fd(pr->pi), FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
make[2]: *** [pagemap.o] Error 1

CC: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>