fix pocomsg.h generation so the target stops rebuilding

The output directory specified by -h should match the RES_SRCS directory.
Changed the RES_SRCS to specify the srcs directory.

The real bug here is that the add_custom_command() DEPENDS rule is never satisfied because this mismatch. The mismatch causes the target to always be out of date. This fix changes nothing about the location of the generated file.