Mark runtargettests as a phony rule

It doesn't produce an output file, so this should be marked as phony.

Test: Fewer warnings when --warn_real_to_phony is enabled for kati
Change-Id: I2c1733603923146ea0981d76f592c281fcbffda1
diff --git a/ b/
index 38c3a3b..af20a2f 100644
--- a/
+++ b/
@@ -17,6 +17,7 @@
 # We su shell because process tests try setting "illegal"
 # uid/gids and expecting failures, but root can legally
 # set those to any value.
+.PHONY: runtargettests
 runtargettests: libbrillo_test
 	adb sync
 	adb shell su shell /data/nativetest/libbrillo_test/libbrillo_test