dahdi: strnicmp() -> strncasecmp()

With commit (af3cd13501 "lib/string.c: remove strnicmp()") [1] dahdi can no
longer call strnicmp directly. strncasecmp was added into lib/string.c in kernel
version 2.6.22 so we'll map calls to strncasecmp to strnicmp for any kernel
before that.

This is necessary to compile against kernels >= 4.0.

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=af3cd13501

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Acked-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>
(cherry picked from commit 1559db9d1a)

include/dahdi/kernel.h

@@ -1502,6 +1502,8 @@ void dahdi_pci_disable_link_state(struct pci_dev *pdev, int state);
 #define list_first_entry(ptr, type, member) \
 	list_entry((ptr)->next, type, member)
 
+#define strncasecmp strnicmp
+
 #ifndef __packed
 #define __packed  __attribute__((packed))
 #endif