utils: Use G_GNUC_CONST instead of G_GNUC_PURE for conversion functions
utils: Use G_GNUC_CONST instead of G_GNUC_PURE for conversion functions

Fixes bug #614629.

jpxiong/gstreamer,magcius/gstreamer,krieger-od/gstreamer,collects/gstreamer,ylatuya/gstreamer,collects/gstreamer,centricular/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,ensonic/gstreamer,Distrotech/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,ahmedammar/platform_external_gst_gstreamer,lubosz/gstreamer,mrchapp/gstreamer,ylatuya/gstreamer,lovebug356/gstreamer,ensonic/gstreamer,krieger-od/gstreamer,lubosz/gstreamer,Distrotech/gstreamer,ahmedammar/platform_external_gst_gstreamer,ylatuya/gstreamer,collects/gstreamer,mrchapp/gstreamer,ensonic/gstreamer,jpakkane/gstreamer,justinjoy/gstreamer,surround-io/gstreamer,magcius/gstreamer,drothlis/gstreamer,magcius/gstreamer,cablelabs/gstreamer,surround-io/gstreamer,krieger-od/gstreamer,surround-io/gstreamer,justinjoy/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,justinjoy/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,Distrotech/gstreamer,jpxiong/gstreamer,collects/gstreamer,cfoch/gstreamer,StreamUtils/gstreamer,drothlis/gstreamer,justinjoy/gstreamer,surround-io/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,drothlis/gstreamer,collects/gstreamer,jpakkane/gstreamer,mrchapp/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,krieger-od/gstreamer,drothlis/gstreamer,justinjoy/gstreamer,krichter722/gstreamer,mparis/gstreamer,centricular/gstreamer,lovebug356/gstreamer,StreamUtils/gstreamer,lubosz/gstreamer,krichter722/gstreamer,mparis/gstreamer,ylatuya/gstreamer,cablelabs/gstreamer,cfoch/gstreamer,shelsonjava/gstreamer,cablelabs/gstreamer,mparis/gstreamer,Lachann/gstreamer,Distrotech/gstreamer,lovebug356/gstreamer,drothlis/gstreamer,magcius/gstreamer,ahmedammar/platform_external_gst_gstreamer,Lachann/gstreamer,jpakkane/gstreamer,cfoch/gstreamer,ahmedammar/platform_external_gst_gstreamer,mparis/gstreamer,centricular/gstreamer,lovebug356/gstreamer,StreamUtils/gstreamer,cablelabs/gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,jpakkane/gstreamer,ahmedammar/platform_external_gst_gstreamer,cfoch/gstreamer,jpxiong/gstreamer,magcius/gstreamer,mparis/gstreamer,ensonic/gstreamer,krichter722/gstreamer,centricular/gstreamer,ylatuya/gstreamer,mrchapp/gstreamer,Lachann/gstreamer,centricular/gstreamer,mrchapp/gstreamer,lubosz/gstreamer,Distrotech/gstreamer,cablelabs/gstreamer