fix: include <strings.h> for strncasecmp

[bonifaido]

Fixes #6.

gcc 15 promotes -Wimplicit-function-declaration to an error, so the two strncasecmp() calls in sigv4.c fail the build. Under -std=c99 (__STRICT_ANSI__) glibc's string.h does not pull in strings.h, where POSIX declares the function.

sigv4.c:277:9: error: implicit declaration of function 'strncasecmp'; did you mean 'strncmp'? [-Wimplicit-function-declaration]

Changes

• sigv4.h: add #include <strings.h>, only inside the standard-headers branch. Kernel builds set SIGV4_SYSTEM_HEADER and take the other branch — strings.h does not exist there, and strncasecmp comes from linux/string.h.
• sigv4.c: cast key.data (unsigned char *) to char * at both call sites, as aws_sigv4_strcmp() already does. Without this, declaring the function just trades the error for two new -Wpointer-sign warnings on the platforms that were already compiling.

Verification

Reproduced and fixed on Ubuntu 26.04 / gcc 15.2.0 (aarch64), matching the reporter's environment.

Hosted build, -Wall -Wextra -std=c99:

	before	after
strncasecmp implicit decl	1 error	gone
other warnings in sigv4.c	7	7 (unchanged)

The 7 remaining are pre-existing -Wpointer-sign on aws_sigv4_string("literal") — string literals passed as const unsigned char *. Untouched here; separate cleanup.

make test → 100%: Checks: 6, Failures: 0, Errors: 0.

Kernel path, compiled through SIGV4_SYSTEM_HEADER="c_system_header.h" with the driver's flags plus -Werror=implicit-function-declaration:

CC [M]  sigv4.o          # clean
nm -u sigv4.o → U strncasecmp    # resolves to the kernel's own