Use GitHub C syntax highlighting on test files

Add a .gitattributes file that tells GitHub to highlight all .function
files as if they were .c files. This aids in reviewing changes to tests.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..ceb59d7
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,2 @@
+# Classify all '.function' files as C for syntax highlighting purposes
+*.function linguist-language=C