CppUTest: fix integer conversion warning in gettid()

[pcolberg]

The cast is required since syscall() provides a generalized interface.

See, e.g. https://github.com/elogind/elogind/blob/06e702c9dafa3ea1dd6df8ee8cb4dcf417a0d442/src/basic/missing_syscall.h#L76-L87

[19/113] Building CXX object lib/CppUTest/CMakeFiles/CppUTest.dir/src/UtestPlatformGcc.cpp.o
../lib/CppUTest/src/UtestPlatformGcc.cpp: In function ‘pid_t gettid()’:
../lib/CppUTest/src/UtestPlatformGcc.cpp:59:50: warning: conversion to ‘pid_t {aka int}’ from ‘long int’ may alter its value [-Wconversion]
 static inline pid_t gettid(void) { return syscall( __NR_gettid ); }
                                           ~~~~~~~^~~~~~~~~~~~~~~

Prerequisite of #85