gitignore: add .idea path

JetBrains IDEs use .idea folder to store configuration files.
To provide more comfort for it's users .idea folder should be ignored.

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>

.gitignore

@@ -26,6 +26,7 @@ CTestTestfile.cmake
 Testing/
 ./build
 /build
+.idea/
 # Ignore Makefile in the stupid way for `tar --exclude-vcs-ignores`
 /Makefile
 ./Makefile