Fixed Linux build with GL 3.1
Fixed Linux build with GL 3.1

bkaradzic/bgfx,fluffyfreak/bgfx,BlueCrystalLabs/bgfx,marco-we/bgfx,BlueCrystalLabs/bgfx,jdryg/bgfx,septag/bgfx,v3n/bgfx,kondrak/bgfx,cyndis/bgfx,Vertexwahn/bgfx,elmindreda/bgfx,janstk/bgfx,ocornut/bgfx,Vertexwahn/bgfx,emoon/bgfx,Synxis/bgfx,0-wiz-0/bgfx,Extrawurst/bgfx,cuavas/bgfx,mendsley/bgfx,cyndis/bgfx,ocornut/bgfx,mcanthony/bgfx,ocornut/bgfx,ming4883/bgfx,aonorin/bgfx,v3n/bgfx,mmicko/bgfx,LSBOSS/bgfx,ktotheoz/bgfx,cyndis/bgfx,Synxis/bgfx,ming4883/bgfx,MikePopoloski/bgfx,LWJGL-CI/bgfx,mcanthony/bgfx,Extrawurst/bgfx,marco-we/bgfx,sergeScherbakov/bgfx,BlueCrystalLabs/bgfx,janstk/bgfx,sergeScherbakov/bgfx,Extrawurst/bgfx,elmindreda/bgfx,LWJGL-CI/bgfx,fluffyfreak/bgfx,attilaz/bgfx,sergeScherbakov/bgfx,MikePopoloski/bgfx,darkimage/bgfx,fluffyfreak/bgfx,cuavas/bgfx,v3n/bgfx,mcanthony/bgfx,bkaradzic/bgfx,bkaradzic/bgfx,emoon/bgfx,mendsley/bgfx,jpcy/bgfx,janstk/bgfx,jdryg/bgfx,kondrak/bgfx,mmicko/bgfx,ktotheoz/bgfx,jdryg/bgfx,jdryg/bgfx,marco-we/bgfx,mendsley/bgfx,attilaz/bgfx,kondrak/bgfx,elmindreda/bgfx,0-wiz-0/bgfx,attilaz/bgfx,septag/bgfx,LWJGL-CI/bgfx,LWJGL-CI/bgfx,LSBOSS/bgfx,jpcy/bgfx,LSBOSS/bgfx,darkimage/bgfx,mmicko/bgfx,jpcy/bgfx,andr3wmac/bgfx,darkimage/bgfx,fluffyfreak/bgfx,aonorin/bgfx,aonorin/bgfx,MikePopoloski/bgfx,bkaradzic/bgfx,ktotheoz/bgfx,Vertexwahn/bgfx,Synxis/bgfx,cuavas/bgfx,septag/bgfx,andr3wmac/bgfx,0-wiz-0/bgfx,ming4883/bgfx,emoon/bgfx,andr3wmac/bgfx,jpcy/bgfx