Fix typos in find_mingw_toolchain

MINGW_GCC equaled $MINGW_ROOT/bin/x86_64-w64-mingw32--gcc

I guess nothing needed it to be correct? MINGW_ROOT was still set OK.

It also printed an error:

/usr/local/google/buildbot/src/android/gcc/ndk/build/tools/prebuilt-common.sh: line 732: [: missing `]'

Specifically, the [ program/builtin fails, so effectively the $MINGW_GCC
existence check was disabled. Fixing the typo enables the check.

Test: ./checkbuild.py --system=windows
Test: ./checkbuild.py --system=windows64
Change-Id: Ida91cebf19adcfb4a0711958bd8eb81b570657a7
diff --git a/build/tools/prebuilt-common.sh b/build/tools/prebuilt-common.sh
index cc4fe04..47ced39 100644
--- a/build/tools/prebuilt-common.sh
+++ b/build/tools/prebuilt-common.sh
@@ -728,9 +728,9 @@
     LINUX_GCC_PREBUILTS=$ANDROID_BUILD_TOP/prebuilts/gcc/linux-x86
     MINGW_ROOT=$LINUX_GCC_PREBUILTS/host/x86_64-w64-mingw32-4.8/
     BINPREFIX=x86_64-w64-mingw32-
-    MINGW_GCC=$MINGW_ROOT/bin/${BINPREFIX}-gcc
-    if [ ! -e "$MINGW_GCC"]; then
-      panic "$MINGW_GCC does not exist"
+    MINGW_GCC=$MINGW_ROOT/bin/${BINPREFIX}gcc
+    if [ ! -e "$MINGW_GCC" ]; then
+        panic "$MINGW_GCC does not exist"
     fi
 
     if [ "$HOST_ARCH" = "x86_64" -a "$TRY64" = "yes" ]; then