test: make the symbols-leak-test executable

test: make the symbols-leak-test executable

Python leaks like crazy when run under valgrind. But if we make the script
executable **and** it has uses the env invocation (i.e. #!/usr/bin/env python3),
the leaks disappear. This is not the case for a shebang of /usr/bin/python3.

Why exactly this is the case I'm not sure but executables we plan to run
should have the exec bit set. So this is a janitor patch with the nice side
effect of fixing our valgrind runs.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>