Drop -I/usr/include/slang to allow detection of <slang.h> vs <slang/slang.h>
---- linux-5.10/tools/build/feature/Makefile.orig 2020-12-13 23:41:30.000000000 +0100
-+++ linux-5.10/tools/build/feature/Makefile 2020-12-17 06:28:25.229860807 +0100
+--- linux-5.11/tools/build/feature/Makefile.orig 2021-02-21 21:43:25.374191668 +0100
++++ linux-5.11/tools/build/feature/Makefile 2021-02-21 21:45:11.760281992 +0100
@@ -90,7 +90,7 @@
###############################
$(OUTPUT)test-all.bin:
-- $(BUILD) -fstack-protector-all -O2 -D_FORTIFY_SOURCE=2 -ldw -lelf -lnuma -lelf -I/usr/include/slang -lslang $(FLAGS_PERL_EMBED) $(FLAGS_PYTHON_EMBED) -DPACKAGE='"perf"' -lbfd -ldl -lz -llzma -lzstd
-+ $(BUILD) -fstack-protector-all -O2 -D_FORTIFY_SOURCE=2 -ldw -lelf -lnuma -lelf -lslang $(FLAGS_PERL_EMBED) $(FLAGS_PYTHON_EMBED) -DPACKAGE='"perf"' -lbfd -ldl -lz -llzma -lzstd
+- $(BUILD) -fstack-protector-all -O2 -D_FORTIFY_SOURCE=2 -ldw -lelf -lnuma -lelf -I/usr/include/slang -lslang $(FLAGS_PERL_EMBED) $(FLAGS_PYTHON_EMBED) -DPACKAGE='"perf"' -lbfd -ldl -lz -llzma -lzstd -lcap
++ $(BUILD) -fstack-protector-all -O2 -D_FORTIFY_SOURCE=2 -ldw -lelf -lnuma -lelf -lslang $(FLAGS_PERL_EMBED) $(FLAGS_PYTHON_EMBED) -DPACKAGE='"perf"' -lbfd -ldl -lz -llzma -lzstd -lcap
$(OUTPUT)test-hello.bin:
$(BUILD)