Protect the TEXTMETRIC struct from redefinition.

diff --git a/include/wingdi.h b/include/wingdi.h
index 0a87237..d2a0f92 100644
--- a/include/wingdi.h
+++ b/include/wingdi.h
@@ -762,6 +762,9 @@
 #define FF_SCRIPT           0x40
 #define FF_DECORATIVE       0x50
 
+#ifndef _TEXTMETRIC_DEFINED
+#define _TEXTMETRIC_DEFINED
+
 typedef struct
 {
     LONG      tmHeight;
@@ -813,6 +816,7 @@
 DECL_WINELIB_TYPE_AW(TEXTMETRIC)
 DECL_WINELIB_TYPE_AW(PTEXTMETRIC)
 DECL_WINELIB_TYPE_AW(LPTEXTMETRIC)
+#endif /* _TEXTMETRIC_DEFINED */
 
 
 typedef struct tagPANOSE