get genmipmap function in the struct

src/gpu/gl/GrGLCreateNullInterface.cpp

Index: src/gpu/gl/GrGLCreateNullInterface.cpp
diff --git a/src/gpu/gl/GrGLCreateNullInterface.cpp b/src/gpu/gl/GrGLCreateNullInterface.cpp
index aa9b93c81307f295521e0ac6912e67a5e86c2936..0d539c959389cd4377c50c6963076ece0c199b51 100644
--- a/src/gpu/gl/GrGLCreateNullInterface.cpp
+++ b/src/gpu/gl/GrGLCreateNullInterface.cpp
@@ -109,6 +109,8 @@ GrGLvoid GR_GL_FUNCTION_TYPE nullGLGenBuffers(GrGLsizei n, GrGLuint* ids) {
     }
 }
 
+GrGLvoid GR_GL_FUNCTION_TYPE nullGLGenerateMipmap(GrGLenum target) {}
+
 GrGLvoid GR_GL_FUNCTION_TYPE nullGLBufferData(GrGLenum target,
                                               GrGLsizeiptr size,
                                               const GrGLvoid* data,
@@ -306,6 +308,7 @@ const GrGLInterface* GrGLCreateNullInterface() {
         interface->fFlush = noOpGLFlush;
         interface->fFrontFace = noOpGLFrontFace;
         interface->fGenBuffers = nullGLGenBuffers;
+        interface->fGenerateMipmap = nullGLGenerateMipmap;
         interface->fGenQueries = noOpGLGenIds;
         interface->fGenTextures = noOpGLGenIds;
         interface->fGenVertexArrays = noOpGLGenIds;