[C++] s390: Fix enum gdb_syscall conversion

Fixes:

 src/gdb/s390-linux-tdep.c: In function ‘gdb_syscall s390_canonicalize_syscall(int, s390_abi_kind)’:
 src/gdb/s390-linux-tdep.c:2622:16: error: invalid conversion from ‘int’ to ‘gdb_syscall’ [-fpermissive]
	  return syscall;
		 ^
 src/gdb/s390-linux-tdep.c:2722:16: error: invalid conversion from ‘int’ to ‘gdb_syscall’ [-fpermissive]
	  return syscall;
		 ^
 src/gdb/s390-linux-tdep.c:2725:24: error: invalid conversion from ‘int’ to ‘gdb_syscall’ [-fpermissive]
	  return syscall + 2;
			 ^
 src/gdb/s390-linux-tdep.c:2728:24: error: invalid conversion from ‘int’ to ‘gdb_syscall’ [-fpermissive]
	  return syscall + 5;
			 ^
 src/gdb/s390-linux-tdep.c:2731:24: error: invalid conversion from ‘int’ to ‘gdb_syscall’ [-fpermissive]
	  return syscall + 6;
			 ^
 src/gdb/s390-linux-tdep.c:2734:24: error: invalid conversion from ‘int’ to ‘gdb_syscall’ [-fpermissive]
	  return syscall + 7;
			 ^

gdb/ChangeLog:
2015-11-19  Simon Marchi  <simon.marchi@ericsson.com>
	    Pedro Alves  <palves@redhat.com>

	* s390-linux-tdep.c (s390_canonicalize_syscall): Add casts and
	intermediate 'int' variable.