gitignore: fix broken build dir glob

.gitignore do not adhere to regex, but their own glob based pattern
matching.

Signed-off-by: Jan Kraemer <jan.kraemer@vector.com>

Author: VJanKraemer

.gitignore

@@ -1,7 +1,7 @@
 # compile commands for clangd  support
 compile_commands.json
 # ignore cmake-generated path directories
-^build*/
+build*/
 _build*/
 install*/
 _install*/