#include <strings.h> for strcasecmp in versit parser

Summary:
The POSIX definition for strcasecmp states that it is in the <strings.h> include file, not <string.h>. This causes the following build error on the musl libc:

/usr/src/packages/user/kcalcore/src/kcalcore-17.08.1/src/versit/vcc.y: In function 'enterAttr':
 /usr/src/packages/user/kcalcore/src/kcalcore-17.08.1/src/versit/vcc.y:485:9: error: implicit definition of function 'strcasecmp' [-Werror=implicit-function-definition]

This patch fixes this error (and a few others later on in the file).

Test Plan: Built and tested on glibc and musl Linux systems; since <strings.h> is a POSIX standard, it is assumed that other Unix-based systems (such as BSD and Solaris) provide it as well.

Reviewers: KDE PIM, winterz

Reviewed By: KDE PIM, winterz

Subscribers: winterz, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8255