use I3__FILE__ for DLOG, leave __FILE__ as is

See also commit 0e752070ac, which broke
source code listings in gdb unless you cd into i3/src. This should give
us best of both :-).

common.mk

@@ -56,6 +56,7 @@ I3_CPPFLAGS += -DMAJOR_VERSION=${MAJOR_VERSION}
 I3_CPPFLAGS += -DMINOR_VERSION=${MINOR_VERSION}
 I3_CPPFLAGS += -DPATCH_VERSION=${PATCH_VERSION}
 I3_CPPFLAGS += -DSYSCONFDIR=\"${SYSCONFDIR}\"
+I3_CPPFLAGS += -DI3__FILE__=__FILE__
 
 
 ## Libraries flags