package/gerbera: fix build with -Os and gcc 9 or 10

For an unknown reason, gerbera fails to build with -Os and gcc 9 or 10
since bump to version 1.8.0 in commit 89745968:

[100%] Linking CXX executable gerbera
/home/giuliobenetti/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-gnu/10.2.0/../../../../x86_64-buildroot-linux-gnu/bin/ld: liblibgerbera.a(content_manager.cc.o): in function `ContentManager::_rescanDirectory(std::shared_ptr<AutoscanDirectory>&, int, std::shared_ptr<GenericTask> const&)':
content_manager.cc:(.text+0xb53b): undefined reference to `std::__shared_ptr<std::filesystem::__cxx11::_Dir, (__gnu_cxx::_Lock_policy)2>::swap(std::__shared_ptr<std::filesystem::__cxx11::_Dir, (__gnu_cxx::_Lock_policy)2>&)'
collect2: error: ld returned 1 exit status

A similar build failure has been reported as gcc bug 91067:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91067

But this bug has been fixed since gcc 9.3 and 10.1 and build failures
are raised with gcc 10.2

To fix this build failure, set optimisation to -O2 if needed

Fixes:
 - http://autobuild.buildroot.org/results/a4ee8ad7ff93939716673b611c7cc3f68dafa3d0



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>