Test for _WIN32 in instead of USE_MINGW.

Bug: http://b/23392815
Test: builds
Change-Id: I7fce8de1f0e2314170cce456cd49346dcee9d134
diff --git a/include/backtrace/BacktraceMap.h b/include/backtrace/BacktraceMap.h
index b80045f..df48dfe 100644
--- a/include/backtrace/BacktraceMap.h
+++ b/include/backtrace/BacktraceMap.h
@@ -19,7 +19,7 @@
 
 #include <stdint.h>
 #include <sys/types.h>
-#ifdef USE_MINGW
+#ifdef _WIN32
 // MINGW does not define these constants.
 #define PROT_NONE 0
 #define PROT_READ 0x1