dovecot-pigeonhole: fix Makefile syntax to match dovecot

dovecot-pigeonhole: fix Makefile syntax to match dovecot

Use $(XARGS) variable instead of plain xargs command, consistent with
the dovecot package implementation. Remove unnecessary space in
EXTRA_DEPENDS version constraint.

Fixes failing error on buildbot:
```
make[3]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.
Makefile:62: *** multiple target patterns.  Stop.
time: package/feeds/packages/pigeonhole/compile#0.52#0.42#1.15
```

Signed-off-by: Josef Schlehofer <redacted>