Make (and verify) public/ files compile under C.

testing/embedder_test.cpp

Index: testing/embedder_test.cpp
diff --git a/testing/embedder_test.cpp b/testing/embedder_test.cpp
index e1473c0473425fe6607b3c7f30180dcbbaa81112..4cf7a8db8a808812bc9ef52a8a1adf5647c70625 100644
--- a/testing/embedder_test.cpp
+++ b/testing/embedder_test.cpp
@@ -166,7 +166,7 @@ int Get_Block(void* param, unsigned long pos, unsigned char* pBuf,
   return 1;
 }
 
-bool Is_Data_Avail(FX_FILEAVAIL* pThis, size_t offset, size_t size) {
+FPDF_BOOL Is_Data_Avail(FX_FILEAVAIL* pThis, size_t offset, size_t size) {
   return true;
 }