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

samples/pdfium_test.cc

Index: samples/pdfium_test.cc
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index a3c4fdf35248629cc828730cfc62e74aba32c0ac..a3cc645db376065400fafd4a7845d487e8e133da 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -413,7 +413,7 @@ 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;
 }