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

Modules/Adapters/OSSIMAdapters/test/otbOssimElevManagerTest4.cxx

@@ -96,7 +96,7 @@ int otbOssimElevManagerTest4(int argc, char* argv[])
 
   std::ofstream file;
   std::cout << outfname << std::endl;
-  file.open(outfname, ios::binary | ios::out);
+  file.open(outfname, std::ios::binary | std::ios::out);
 
   file.write(reinterpret_cast<char*>(image), sizeof(double) * size[0] * size[1]);
   file.close();