fix compile in api 24

 * Fixes the following error
   bootable/recovery/twrp.cpp:173:18: error: use of undeclared identifier 'android'

                                other_prop = android::base::Trim(other_prop);

                                             ^

   bootable/recovery/twrp.cpp:174:20: error: use of undeclared identifier 'android'

                                current_prop = android::base::Trim(current_prop);

Change-Id: I1d82cc68d99e5b42cd32a6fb96e9331681f56ba2
Signed-off-by: Mohd Faraz <androiabledroid@gmail.com>
diff --git a/Android.mk b/Android.mk
index af49367..2725a0b 100755
--- a/Android.mk
+++ b/Android.mk
@@ -143,6 +143,7 @@
     LOCAL_SHARED_LIBRARIES += libstlport
     LOCAL_CFLAGS += -DTW_NO_SHA2_LIBRARY
 endif
+LOCAL_CFLAGS += -DSDK_VERSION=$(PLATFORM_SDK_VERSION)
 ifeq ($(shell test $(PLATFORM_SDK_VERSION) -lt 25; echo $$?),0)
     LOCAL_CFLAGS += -DUSE_OLD_BASE_INCLUDE
 endif