sysutils/bareos-server: Fix build with GCC-based architectures

Add USES=compiler:c++11-lang:
CMake Error in core/src/plugins/filed/CMakeLists.txt:

Target "bpipe-fd" requires the language dialect "CXX11" (with compiler
extensions), but CMake does not know the compile flags to use to enable it.

Approved by: mentors (implicit approval)