elf.h: add NT_ARM_PAC_ENABLED_KEYS from linux v5.13

see

  linux commit 201698626fbca1cf1a3b686ba14cf2a056500716
  arm64: Introduce prctl(PR_PAC_{SET,GET}_ENABLED_KEYS)

include/elf.h

@@ -687,6 +687,7 @@ typedef struct {
 #define NT_ARM_PACA_KEYS	0x407
 #define NT_ARM_PACG_KEYS	0x408
 #define NT_ARM_TAGGED_ADDR_CTRL	0x409
+#define NT_ARM_PAC_ENABLED_KEYS	0x40a
 #define NT_METAG_CBUF	0x500
 #define NT_METAG_RPIPE	0x501
 #define NT_METAG_TLS	0x502