cmake: fix finding of headers for RootHistogramSink
GaudiCommonSvc/src/HistogramPersistencySvc/RootHistogramSink.cpp fails to find HistogramPersistencySvc/RootHistogramSinkBase.h since the corresponding include directory does not appear to get added to the build.
Forwarded from https://github.com/key4hep/Gaudi/pull/2/files
It seems like there is a possibility to build this successfully(?), but not with the default cmake arguments. I also checked !1401 (merged) to see if this is fixed already there, but it doesn't seem to be the case.
The build error without this fix is:
/home/tmadlener/work/Gaudi/GaudiCommonSvc/src/HistogramPersistencySvc/RootHistogramSink.cpp:14:10: fatal error: HistogramPersistencySvc/RootHistogramSinkBase.h: No such file or directory
14 | #include <HistogramPersistencySvc/RootHistogramSinkBase.h>
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~