Add Google-style StringPrintf.

Change-Id: If333c28b8ed047346a2dd85e591c4bedbab874cc
diff --git a/libutils/Android.mk b/libutils/Android.mk
index 701fe0e..7bff14e 100644
--- a/libutils/Android.mk
+++ b/libutils/Android.mk
@@ -41,6 +41,7 @@
 	VectorImpl.cpp \
 	file.cpp \
 	misc.cpp \
+	stringprintf.cpp \
 
 host_commonCflags := -DLIBUTILS_NATIVE=1 $(TOOL_CFLAGS) -Werror