testsuite: Fix build with gcc < 15

testsuite: Fix build with gcc < 15

Variable declaration inside a switch case works with gcc 15, but fails
pretty much everywhere else with a message like below:

	../testsuite/init_module.c: In function ‘syscall’:
	../testsuite/init_module.c:343:3: error: a label can only be part of a statement and a declaration is not a statement
	  343 |   const char *args;
	      |   ^~~~~

Add an empty statement to fix it.

Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/390