openexr: Fix build with OpenEXR 3
Add missing include to fix build errors:
../subprojects/gst-plugins-bad/ext/openexr/gstopenexrdec.cpp: At global scope:
../subprojects/gst-plugins-bad/ext/openexr/gstopenexrdec.cpp:45:11: error: ‘Int64’ does not name a type
45 | virtual Int64 tellg ();et cetera.
The header file is also present in OpenEXR 2 so should not cause issues, but I did not test this explicitly.
