执行

sudo make -C /usr/src/linux-headers-5.11.0-1021-raspi SUBDIRS=$PWD modules
make: Entering directory '/usr/src/linux-headers-5.11.0-1021-raspi'

报错

make: Entering directory '/usr/src/linux-headers-5.11.0-1021-raspi'
  SYNC    include/config/auto.conf.cmd
  YACC    scripts/kconfig/parser.tab.[ch]
  HOSTCC  scripts/kconfig/lexer.lex.o
  HOSTCC  scripts/kconfig/parser.tab.o
  HOSTCC  scripts/kconfig/preprocess.o
  HOSTCC  scripts/kconfig/symbol.o
  HOSTCC  scripts/kconfig/util.o
  HOSTLD  scripts/kconfig/conf
  HOSTCC  scripts/dtc/dtc.o
  HOSTCC  scripts/dtc/flattree.o
  HOSTCC  scripts/dtc/fstree.o
  HOSTCC  scripts/dtc/data.o
  HOSTCC  scripts/dtc/livetree.o
  HOSTCC  scripts/dtc/treesource.o
  HOSTCC  scripts/dtc/srcpos.o
  HOSTCC  scripts/dtc/checks.o
  HOSTCC  scripts/dtc/util.o
  LEX     scripts/dtc/dtc-lexer.lex.c
  YACC    scripts/dtc/dtc-parser.tab.[ch]
  HOSTCC  scripts/dtc/dtc-lexer.lex.o
  HOSTCC  scripts/dtc/dtc-parser.tab.o
  HOSTLD  scripts/dtc/dtc
  HOSTCC  scripts/genksyms/genksyms.o
  YACC    scripts/genksyms/parse.tab.[ch]
  HOSTCC  scripts/genksyms/parse.tab.o
  LEX     scripts/genksyms/lex.lex.c
  HOSTCC  scripts/genksyms/lex.lex.o
  HOSTLD  scripts/genksyms/genksyms
  HOSTCC  scripts/selinux/genheaders/genheaders
scripts/selinux/genheaders/genheaders.c:18:10: fatal error: classmap.h: No such file or directory
   18 | #include "classmap.h"
      |          ^~~~~~~~~~~~
compilation terminated.
make[3]: *** [scripts/Makefile.host:95: scripts/selinux/genheaders/genheaders] Error 1
make[2]: *** [scripts/Makefile.build:519: scripts/selinux/genheaders] Error 2
make[1]: *** [scripts/Makefile.build:519: scripts/selinux] Error 2
make: *** [Makefile:1225: scripts] Error 2
make: Leaving directory '/usr/src/linux-headers-5.11.0-1021-raspi'

参考 Bug #1895470 “fatal error: classmap.h: No such file or directory...” : Bugs : linux-kernel-headers package : Ubuntu theway (lexueye) 的回答，Ubuntu 20.04 起不再支持 SUBDIR=，应改为 M=：

sudo make -C /usr/src/linux-headers-5.11.0-1021-raspi M=$PWD modules
make: Entering directory '/usr/src/linux-headers-5.11.0-1021-raspi'

即可成功编译。