backports: backport-3.15: Include fix

backports: backport-3.15: Include fix

In this version the correct linux string.h is included.

Include string.h to avoid compile issues if warnings are treated as errors:

backports-3.14-1/compat/backport-3.15.c: In function 'backport_devm_kstrdup':
backports-3.14-1/compat/backport-3.15.c:33:2: error: implicit declaration of function 'strlen' [-Werror=implicit-function-declaration]
   size = strlen(s) + 1;
   ^
backports-3.14-1/compat/backport-3.15.c:33:9: warning: incompatible implicit declaration of built-in function 'strlen' [enabled by default]
   size = strlen(s) + 1;
          ^
backports-3.14-1/compat/backport-3.15.c:36:3: error: implicit declaration of function 'memcpy' [-Werror=implicit-function-declaration]
    memcpy(buf, s, size);
    ^
backports-3.14-1/compat/backport-3.15.c:36:3: warning: incompatible implicit declaration of built-in function 'memcpy' [enabled by default]

Signed-off-by: Richard Röjfors <richard@puffinpack.se>
[refreshed patch on current master]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>