fdo#68849: Add header guards to all include files

Added header guard to files in directories forms/ and fpicker/

Change-Id: I5a3a2b1f2aa81d048959cb3223e03ac985539f33
Reviewed-on: https://gerrit.libreoffice.org/9557
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>