libdpkg: Add missing <string.h> header

These are currently shadowed by the <string.h> inclusion from
<compat/strnlen.h>.

lib/dpkg/error.c

@@ -22,6 +22,7 @@
 #include <compat.h>
 
 #include <errno.h>
+#include <string.h>
 #include <stdlib.h>
 
 #include <dpkg/dpkg.h>

lib/dpkg/namevalue.c

@@ -22,6 +22,7 @@
 #include <compat.h>
 
 #include <stddef.h>
+#include <string.h>
 
 #include <dpkg/namevalue.h>
 

lib/dpkg/trignote.c

@@ -23,6 +23,8 @@
 #include <config.h>
 #include <compat.h>
 
+#include <string.h>
+
 #include <dpkg/dpkg.h>
 #include <dpkg/dpkg-db.h>
 #include <dpkg/pkg.h>

lib/dpkg/varbuf.h

@@ -24,6 +24,7 @@
 
 #include <stddef.h>
 #include <stdarg.h>
+#include <string.h>
 
 #include <dpkg/macros.h>