Fix tzHour usage on systems where char is unsigned.

BUG=chromium:395832
R=bo_xu@foxitsoftware.com

Review URL: https://codereview.chromium.org/478353002

fpdfsdk/include/fsdk_baseannot.h

diff --git a/fpdfsdk/include/fsdk_baseannot.h b/fpdfsdk/include/fsdk_baseannot.h
index 50a6e1e..797c3f1 100644
--- a/fpdfsdk/include/fsdk_baseannot.h
+++ b/fpdfsdk/include/fsdk_baseannot.h
@@ -53,7 +53,7 @@
 		FX_BYTE		hour;
 		FX_BYTE		minute;
 		FX_BYTE		second;
-		FX_CHAR		tzHour;
+		FX_INT8 	tzHour;
 		FX_BYTE		tzMinute;
 	}dt;
 };