RFR: 8252060: gstreamer fails to build with gcc 10

[Alexander Matveev]

Fixed by defining GST_API as GST_EXPORT for gcc compiler as per Kevin proposed solution. On Windows we do not need to
define GST_API, since we using .def file to export symbols.

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Commit messages:
 - 8252060: gstreamer fails to build with gcc 10

Changes: https://git.openjdk.java.net/jfx/pull/287/files
 Webrev: https://webrevs.openjdk.java.net/jfx/287/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252060
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/287.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/287/head:pull/287

PR: https://git.openjdk.java.net/jfx/pull/287