Fix gcc8 compilation

For some reason, missing "std::" prevent compilation under debian/testing with gcc-8

[ 28%] Building CXX object Modules/ThirdParty/OssimPlugins/test/CMakeFiles/OTBossimSentinel1ModelTest.dir/ossimSentinel1ModelTest.cpp.o
/home/otb-user/src/otb/Modules/ThirdParty/OssimPlugins/test/ossimSentinel1ModelTest.cpp: In function ‘int main(int, char**)’:
/home/otb-user/src/otb/Modules/ThirdParty/OssimPlugins/test/ossimSentinel1ModelTest.cpp:51:4: error: ‘string’ was not declared in this scope
    string annotationXml;
...
[ 28%] Building CXX object Modules/Adapters/OSSIMAdapters/test/CMakeFiles/otbOSSIMAdaptersTestDriver.dir/otbOssimElevManagerTest4.cxx.o
/home/otb-user/src/otb/Modules/Adapters/OSSIMAdapters/test/otbOssimElevManagerTest4.cxx: In function ‘int otbOssimElevManagerTest4(int, char**)’:
/home/otb-user/src/otb/Modules/Adapters/OSSIMAdapters/test/otbOssimElevManagerTest4.cxx:99:23: error: ‘ios’ has not been declared