build: include ptrace.h after regs.h

Restore the order of including <sys/reg.h> and <linux/ptrace.h>
headers that was inadvertently changed by commit v4.9-280-g5503dd2.

This should fix build on platforms like m68k where some constants
are defined simultaneously as enums by <sys/reg.h> and as macros
by <asm/ptrace.h>.

* process.c: Include "ptrace.h" after "regs.h".
* signal.c: Likewise.
* syscall.c: Likewise.
* util.c: Likewise.
diff --git a/signal.c b/signal.c
index 30358a1..e77fd4e 100644
--- a/signal.c
+++ b/signal.c
@@ -34,8 +34,8 @@
 #include "defs.h"
 #include <fcntl.h>
 
-#include "ptrace.h"
 #include "regs.h"
+#include "ptrace.h"
 
 #if defined(SPARC) || defined(SPARC64) || defined(MIPS)
 typedef struct {