Build all bionic test files -Werror and fix our one warning.

Change-Id: I62bb0528fd1bcb9aa4c61e44c78158a592690fc7
diff --git a/tests/Android.mk b/tests/Android.mk
index ab4409f..278511b 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -244,8 +244,8 @@
     dlext_test.cpp \
     dlfcn_test.cpp \
 
-bionic-unit-tests_cppflags := \
-    $(test_cppflags)
+bionic-unit-tests_cflags := $(test_cflags)
+bionic-unit-tests_cppflags := $(test_cppflags)
 
 bionic-unit-tests_ldflags := \
     -Wl,--export-dynamic \
@@ -301,8 +301,8 @@
 bionic-unit-tests-glibc_ldlibs := \
     -lrt -ldl \
 
-bionic-unit-tests-glibc_cppflags := \
-    $(test_cppflags)
+bionic-unit-tests-glibc_cflags := $(test_cflags)
+bionic-unit-tests-glibc_cppflags := $(test_cppflags)
 
 module := bionic-unit-tests-glibc
 module_tag := optional