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.

Basically, RES_SRCS didnt match with the generation directory. I wasnt 100% sure about the intention -- I think pocomsg.h was meant to be a private header. In that case, this is probably the right fix.