science/orthanc-mysql: unbreak build after 4aec6e124381

In file included from MySQL/UnitTests/UnitTestsMain.cpp:32:
In file included from MySQL/UnitTests/../../Framework/Plugins/IndexUnitTests.h:32:
In file included from /usr/local/include/gtest/gtest.h:64:
In file included from /usr/local/include/gtest/gtest-assertion-result.h:46:
In file included from /usr/local/include/gtest/gtest-message.h:57:
/usr/local/include/gtest/internal/gtest-port.h:270:2: error: C++ versions less than C++14 are not supported.
#error C++ versions less than C++14 are not supported.

^

Reported by: pkg-fallout