ARM: fix perf build with uclibc toolchains

libio.h is not provided by uClibc, in order to be able to test the
definition of __UCLIBC__ we need to include stdlib.h, which also
includes stddef.h, providing the definition of 'NULL'.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

tools/perf/arch/arm/util/dwarf-regs.c

@@ -8,7 +8,10 @@
  * published by the Free Software Foundation.
  */
 
+#include <stdlib.h>
+#ifndef __UCLIBC__
 #include <libio.h>
+#endif
 #include <dwarf-regs.h>
 
 struct pt_regs_dwarfnum {