simplify tools/xsan_build now that make runs ninja

BUG=skia:
R=borenet@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/257613004

git-svn-id: http://skia.googlecode.com/svn/trunk@14356 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/Makefile b/Makefile
index a51d6da..1b15a74 100644
--- a/Makefile
+++ b/Makefile
@@ -42,6 +42,7 @@
 VALID_TARGETS := \
                  bench \
                  debugger \
+                 dm \
                  everything \
                  gm \
                  most \