Consolidate error.h inclusion in system.h

error.h isn't standard and so isn't part of the musl C library.
To easy future porting, consolidate the inclusion of error.h into system.h.

https://sourceware.org/bugzilla/show_bug.cgi?id=21008

Signed-off-by: Ross Burton <ross.burton@intel.com>
diff --git a/src/arlib.c b/src/arlib.c
index e0839aa..778e087 100644
--- a/src/arlib.c
+++ b/src/arlib.c
@@ -21,7 +21,6 @@
 #endif
 
 #include <assert.h>
-#include <error.h>
 #include <gelf.h>
 #include <inttypes.h>
 #include <libintl.h>
@@ -31,6 +30,7 @@
 
 #include <libeu.h>
 
+#include "system.h"
 #include "arlib.h"