Add Linguist language override for include files (#1079)

Add a `linguist-language` Git attribute that matches the files at `include/gsl` and sets their language to C++. Without this override, these files are not detected as C++ code by [GitHub's Linguist](https://github.com/github/linguist) (likely due to lacking any file extensions).

This will cause GitHub.com to count these files correctly as C++ code and do syntax highlighting when browsing the repo.

**Linguist breakdown results**

Before:

<details>


```
$ docker run --rm -v $(pwd):$(pwd) -w $(pwd) -t linguist github-linguist --breakdown
91.57%  152148     C++
8.43%   14011      CMake

CMake:
CMakeLists.txt
cmake/gsl_functions.cmake
cmake/gsl_install.cmake
include/CMakeLists.txt
tests/CMakeLists.txt

C++:
tests/algorithm_tests.cpp
tests/assertion_tests.cpp
tests/at_tests.cpp
tests/byte_tests.cpp
tests/deathTestCommon.h
tests/no_exception_ensure_tests.cpp
tests/notnull_tests.cpp
tests/owner_tests.cpp
tests/span_compatibility_tests.cpp
tests/span_ext_tests.cpp
tests/span_tests.cpp
tests/strict_notnull_tests.cpp
tests/utils_tests.cpp
```

</details>

After:

<details>

```
$ docker run --rm -v $(pwd):$(pwd) -w $(pwd) -t linguist github-linguist --breakdown
94.16%  225699     C++
5.84%   14011      CMake

CMake:
CMakeLists.txt
cmake/gsl_functions.cmake
cmake/gsl_install.cmake
include/CMakeLists.txt
tests/CMakeLists.txt

C++:
include/gsl/algorithm
include/gsl/assert
include/gsl/byte
include/gsl/gsl
include/gsl/gsl_algorithm
include/gsl/gsl_assert
include/gsl/gsl_byte
include/gsl/gsl_narrow
include/gsl/gsl_util
include/gsl/narrow
include/gsl/pointers
include/gsl/span
include/gsl/span_ext
include/gsl/string_span
include/gsl/util
tests/algorithm_tests.cpp
tests/assertion_tests.cpp
tests/at_tests.cpp
tests/byte_tests.cpp
tests/deathTestCommon.h
tests/no_exception_ensure_tests.cpp
tests/notnull_tests.cpp
tests/owner_tests.cpp
tests/span_compatibility_tests.cpp
tests/span_ext_tests.cpp
tests/span_tests.cpp
tests/strict_notnull_tests.cpp
tests/utils_tests.cpp
```
</details>

.gitattributes

@@ -0,0 +1 @@
+include/gsl/* linguist-language=C++