dynamic_debug: use printk(KERN_WARNING..) in stub function

drivers/infiniband/ulp/srp/ib_srp.c #defines pr_fmt() PFX fmt, but PFX
is not #defined until after <linux/*> headers are included.

This results in a bad expansion of the pr_warn() in the stub function.

2084c2084
<   printk("<4>" PFX "dyndbg supported only in " "CONFIG_DYNAMIC_DEBUG builds\n")

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>