x86: suppress trivial sparse signedness warnings

Could just as easily change the three casts to cast to the correct
type...this patch changes the type of ftrace_nop instead.

Supresses sparse warnings:

 arch/x86/kernel/ftrace.c:157:14: warning: incorrect type in assignment (different signedness)
 arch/x86/kernel/ftrace.c:157:14:    expected long *static [toplevel] ftrace_nop
 arch/x86/kernel/ftrace.c:157:14:    got unsigned long *<noident>
 arch/x86/kernel/ftrace.c:161:14: warning: incorrect type in assignment (different signedness)
 arch/x86/kernel/ftrace.c:161:14:    expected long *static [toplevel] ftrace_nop
 arch/x86/kernel/ftrace.c:161:14:    got unsigned long *<noident>
 arch/x86/kernel/ftrace.c:165:14: warning: incorrect type in assignment (different signedness)
 arch/x86/kernel/ftrace.c:165:14:    expected long *static [toplevel] ftrace_nop
 arch/x86/kernel/ftrace.c:165:14:    got unsigned long *<noident>

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/ftrace.c

@@ -22,7 +22,7 @@
 
 
 /* Long is fine, even if it is only 4 bytes ;-) */
-static long *ftrace_nop;
+static unsigned long *ftrace_nop;
 
 union ftrace_code_union {
 	char code[MCOUNT_INSN_SIZE];