add .gitattributes for github-linguist

this is intended to improve GH statistics of the project, as written,
it changed from:

```
$ git-linguist stats -c $(git rev-parse HEAD^)
{"CMake":12508,"Makefile":7147,"Shell":12202,"M4":7699,"C":534790,"C++":26882,"Smarty":634,"Cuda":180694}
```
to
```
$ git-linguist stats -c $(git rev-parse HEAD)
{"CMake":12508,"Makefile":7147,"Shell":12202,"M4":7699,"C":561672,"Smarty":634,"Cuda":180694}
````

Author: MartinPulec

.gitattributes

@@ -0,0 +1,7 @@
+*.c linguist-language=C
+*.h linguist-language=C
+*.cpp linguist-language=C++
+*.hpp linguist-language=C++
+*.cu linguist-language=CUDA
+*.cuh linguist-language=CUDA
+src/gpujpeg_colorspace.h linguist-language=CUDA