makepanda: don't use pkg-config for assimp

It seems that they removed the -I option, which we need to build with.  This commit is intended to fix the Ubuntu Bionic build.

makepanda/makepanda.py

@@ -828,7 +828,7 @@ if (COMPILER=="GCC"):
         SmartPkgEnable("EIGEN",     "eigen3",    (), ("Eigen/Dense",), target_pkg = 'ALWAYS')
         SmartPkgEnable("ARTOOLKIT", "",          ("AR"), "AR/ar.h")
         SmartPkgEnable("FCOLLADA",  "",          ChooseLib(fcollada_libs, "FCOLLADA"), ("FCollada", "FCollada/FCollada.h"))
-        SmartPkgEnable("ASSIMP",    "assimp", ("assimp"), "assimp")
+        SmartPkgEnable("ASSIMP",    "",          ("assimp"), "assimp")
         SmartPkgEnable("FFMPEG",    ffmpeg_libs, ffmpeg_libs, ("libavformat/avformat.h", "libavcodec/avcodec.h", "libavutil/avutil.h"))
         SmartPkgEnable("SWSCALE",   "libswscale", "libswscale", ("libswscale/swscale.h"), target_pkg = "FFMPEG", thirdparty_dir = "ffmpeg")
         SmartPkgEnable("SWRESAMPLE","libswresample", "libswresample", ("libswresample/swresample.h"), target_pkg = "FFMPEG", thirdparty_dir = "ffmpeg")