Upgrade Windows default build version from Vista to 7

This is needed to build libicui18n on Windows, as it depends on APIs
created in Windows 7 (ResolveLocaleName).

Test: sdk build
Change-Id: If64510a262f7f1d4d356b1a9960ceea114cfa78a
diff --git a/cc/config/x86_windows_host.go b/cc/config/x86_windows_host.go
index e23c0a0..dfdd40c 100644
--- a/cc/config/x86_windows_host.go
+++ b/cc/config/x86_windows_host.go
@@ -33,9 +33,9 @@
 
 		// Use C99-compliant printf functions (%zd).
 		"-D__USE_MINGW_ANSI_STDIO=1",
-		// Admit to using >= Vista. Both are needed because of <_mingw.h>.
-		"-D_WIN32_WINNT=0x0600",
-		"-DWINVER=0x0600",
+		// Admit to using >= Windows 7. Both are needed because of <_mingw.h>.
+		"-D_WIN32_WINNT=0x0601",
+		"-DWINVER=0x0601",
 		// Get 64-bit off_t and related functions.
 		"-D_FILE_OFFSET_BITS=64",