Adapt to different `C_EXP_HDR` settings
I missed this one in my earlier trawl of users of
C_EXP_HDR since it only uses it on its HAL branch.
BuildSys-7_60 adjusts the default setting of C_EXP_HDR so that it doesn't include a trailing .h for native builds. Since we use the value of this variable in one of our rules, adapt accordingly (note that this implementation will work with both old and new BuildSys).