mm-video: define log2f only for jb_mr1

We only need this defined for JB MR1 as other builds may have it
defined in math.h.

Change-Id: I47549854d655815471fa786ca091e5cb4231f353
diff --git a/mm-video/vidc/vdec/Android.mk b/mm-video/vidc/vdec/Android.mk
index d6840c9..1024a84 100755
--- a/mm-video/vidc/vdec/Android.mk
+++ b/mm-video/vidc/vdec/Android.mk
@@ -73,6 +73,10 @@
 libOmxVdec-def += -DUSE_ION
 endif
 
+ifneq ($(call is-platform-sdk-version-at-least,18),true)
+libOmxVdec-def += -DANDROID_JELLYBEAN_MR1=1
+endif
+
 vdec-inc          = $(TARGET_OUT_INTERMEDIATES)/KERNEL_OBJ/usr/include
 
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