libphosh: Add explicit dependency on generated headers

This should fix spurious build failures like

  In file included from ../src/emergency-calls-manager.h:11,
                   from ../src/emergency-calls-manager.c:14:
  ../src/call.h:8:10: fatal error: dbus/calls-dbus.h: No such file or directory
      8 | #include "dbus/calls-dbus.h"
        |          ^~~~~~~~~~~~~~~~~~~
  compilation terminated.

as seen in !1758 (merged)

Signed-off-by: Guido Günther agx@sigxcpu.org