Include <linux/ptrace.h> regardless of <sys/reg.h> existence

This fixes compilation with musl libc.
This approach was already used in process.c, so I assume it is safe.

* signal.c: Move [HAVE_LINUX_PTRACE_H] code out of [HAVE_SYS_REG_H] check.
* syscall.c: Likewise.
* util.c: Likewise.

Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>
diff --git a/syscall.c b/syscall.c
index e74881d..135221b 100644
--- a/syscall.c
+++ b/syscall.c
@@ -37,7 +37,9 @@
 
 #ifdef HAVE_SYS_REG_H
 # include <sys/reg.h>
-#elif defined(HAVE_LINUX_PTRACE_H)
+#endif
+
+#ifdef HAVE_LINUX_PTRACE_H
 # undef PTRACE_SYSCALL
 # ifdef HAVE_STRUCT_IA64_FPREG
 #  define ia64_fpreg XXX_ia64_fpreg