headers_check fix: powerpc, elf.h

fix the following 'make headers_check' warning:

  usr/include/asm-powerpc/elf.h:5: include of <linux/types.h> is preferred over <asm/types.h>
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>

arch/powerpc/include/asm/elf.h

@@ -7,7 +7,7 @@
 #include <asm/string.h>
 #endif
 
-#include <asm/types.h>
+#include <linux/types.h>
 #include <asm/ptrace.h>
 #include <asm/cputable.h>
 #include <asm/auxvec.h>