[gdb/testsuite] Fix missing return type in gdb.linespec/break-asm-file.c

[gdb/testsuite] Fix missing return type in gdb.linespec/break-asm-file.c

On fedora rawhide, when running test-case gdb.linespec/break-asm-file.exp, I
get:
...
gdb compile failed, break-asm-file.c:21:8: error: \
  return type defaults to 'int' [-Wimplicit-int]
   21 | static func()
      |        ^~~~
...

Fix this by adding the missing return type.

Tested on aarch64-linux.

Approved-By: John Baldwin <jhb@FreeBSD.org>