sw_isr_table: Add spaces to fix compiler warning

When using clang to build a C++ file that includes this file, it warns:

include/zephyr/sw_isr_table.h:195:28: error: invalid suffix on literal;
C++11 requires a space between literal and identifier
[-Wreserved-user-defined-literal]
         "." Z_STRINGIFY(prefix)"."file"." Z_STRINGIFY(counter)

Signed-off-by: Tom Hughes <tomhughes@chromium.org>