Add openjpeg pre-processor OPJ_STATIC to pdfium.gyp

Openjpeg removed the definition of OPJ_STATIC in openjpeg.h.
This change is merged in https://pdfium.googlesource.com/pdfium/+/d53e6fdb0a86ca1ddb12876a60f7f2d7508b5349
So need to add OPJ_STATIC to pdfium.gyp

BUG=None
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/624043002
diff --git a/pdfium.gyp b/pdfium.gyp
index 5c5b9ca..606925c 100644
--- a/pdfium.gyp
+++ b/pdfium.gyp
@@ -8,6 +8,7 @@
       '_FXFT_VERSION_=2501',
       '_FPDFSDK_LIB',
       '_NO_GDIPLUS_',  # workaround text rendering issues on Windows
+      'OPJ_STATIC',
     ],
     'conditions': [
       ['pdf_use_skia==1', {