Fix build on uClibc

* defs.h [LINUX]: Define PTRACE_GETEVENTMSG macro.
Patch by Douglas Mencken <dougmencken@gmail.com>.
diff --git a/defs.h b/defs.h
index 7a7a071..0f516b6 100644
--- a/defs.h
+++ b/defs.h
@@ -312,6 +312,9 @@
 # ifndef PTRACE_SETOPTIONS
 #  define PTRACE_SETOPTIONS	0x4200
 # endif
+# ifndef PTRACE_GETEVENTMSG
+#  define PTRACE_GETEVENTMSG	0x4201
+# endif
 # ifndef PTRACE_O_TRACEFORK
 #  define PTRACE_O_TRACEFORK	0x00000002
 # endif