openexr: Fix some warnings

openexr: Fix some warnings

```
../subprojects/gst-plugins-bad/ext/openexr/gstopenexrdec.cpp:46:24: warning: ‘Imf_3_1::Int64’ is deprecated: use uint64_t [-Wdeprecated-declarations]
   46 |   virtual Int64 tellg ();
      |                        ^
In file included from ../subprojects/gst-plugins-bad/ext/openexr/gstopenexrdec.cpp:32:
/usr/include/OpenEXR/ImfInt64.h:23:32: note: declared here
   23 | typedef IMATH_NAMESPACE::Int64 Int64;
      |                                ^~~~~
../subprojects/gst-plugins-bad/ext/openexr/gstopenexrdec.cpp:47:32: warning: ‘Imf_3_1::Int64’ is deprecated: use uint64_t [-Wdeprecated-declarations]
   47 |   virtual void seekg (Int64 pos);
      |                                ^
In file included from ../subprojects/gst-plugins-bad/ext/openexr/gstopenexrdec.cpp:32:
/usr/include/OpenEXR/ImfInt64.h:23:32: note: declared here
   23 | typedef IMATH_NAMESPACE::Int64 Int64;
      |                                ^~~~~
../subprojects/gst-plugins-bad/ext/openexr/gstopenexrdec.cpp:67:26: warning: ‘Imf_3_1::Int64’ is deprecated: use uint64_t [-Wdeprecated-declarations]
   67 | Int64 MemIStream::tellg ()
      |                          ^
In file included from ../subprojects/gst-plugins-bad/ext/openexr/gstopenexrdec.cpp:32:
/usr/include/OpenEXR/ImfInt64.h:23:32: note: declared here
   23 | typedef IMATH_NAMESPACE::Int64 Int64;
      |                                ^~~~~
../subprojects/gst-plugins-bad/ext/openexr/gstopenexrdec.cpp:73:29: warning: ‘Imf_3_1::Int64’ is deprecated: use uint64_t [-Wdeprecated-declarations]
   73 | MemIStream::seekg (Int64 pos)
      |                             ^
In file included from ../subprojects/gst-plugins-bad/ext/openexr/gstopenexrdec.cpp:32:
/usr/include/OpenEXR/ImfInt64.h:23:32: note: declared here
   23 | typedef IMATH_NAMESPACE::Int64 Int64;
      |                                ^~~~~
```

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1977>