memblock tests: fix warning ‘struct seq_file’ declared inside parameter list

memblock tests: fix warning ‘struct seq_file’ declared inside parameter list

Building memblock tests produces the following warning:

cc -I. -I../../include -Wall -O2 -fsanitize=address -fsanitize=undefined -D CONFIG_PHYS_ADDR_T_64BIT   -c -o main.o main.c
In file included from tests/common.h:9,
                 from tests/basic_api.h:5,
                 from main.c:2:
./linux/memblock.h:601:50: warning: ‘struct seq_file’ declared inside parameter list will not be visible outside of this definition or declaration
  601 | static inline void memtest_report_meminfo(struct seq_file *m) { }
      |                                                  ^~~~~~~~

Add declaration of 'struct seq_file' to tools/include/linux/seq_file.h
to fix it.

Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>