src/signature.c: include string.h
src/signature.c: include string.h

We use strstr() but do not include string.h explicitly.

In 98fd6773c4a11738120c605dffdc1fe34ac9be0e, the strstr() method call
was moved from bundle.c to signature.c without porting the include, too.

Leads to compilation warnings with older toolchains:

| [..] warning: incompatible implicit declaration of built-in function 'strstr'

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

jluebbe/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,rauc/rauc,rauc/rauc,ejoerns/rauc