flopen: Add missing <fcntl.h> include

flopen: Add missing <fcntl.h> include

Commit 993828d84ee (Add flopenat() function from FreeBSD) dropped the
fcntl.h header. This breaks the build with musl libc:

flopen.c: In function ‘vflopenat’:
flopen.c:60:14: error: ‘O_CREAT’ undeclared (first use in this function)
  if (flags & O_CREAT) {
              ^~~~~~~

Restore the fcntl.h header include to fix the build.

Fixes: commit 993828d84eed0468c6c15b2818e534e6b134b8e4
Submitted-also-by: parazyd <parazyd@dyne.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Guillem Jover <guillem@hadrons.org>