master branch fails to build because of compiler warning.

[Lingyu Zhu]

Hi, I was trying to build git master tip on freebsd 12.2 release (clang 10.0.1)
The compiler output following message:

make cross-target
(cd microcode && make all)
cc -DHAVE_CONFIG_H -DMIT_SCHEME
-DDEFAULT_LIBRARY_PATH=\"/usr/local/lib/mit-scheme-x86-64-12.0.50\"
-I. -I. -O3  -frounding-math -fno-builtin-floor -Wall -Wempty-body
-Wignored-qualifiers -Wimplicit-fallthrough
-Wmissing-field-initializers -Wnested-externs -Woverride-init
-Wpointer-arith -Wredundant-decls -Wshift-negative-value
-Wstrict-prototypes -Wtype-limits -Wundef -Wuninitialized
-Wwrite-strings -Werror -o interp.o -c interp.c
interp.c:831:5: error: unannotated fall-through between switch labels
[-Werror,-Wimplicit-fallthrough]
    case RC_INTERNAL_APPLY:
    ^
interp.c:831:5: note: insert '__attribute__((fallthrough));' to
silence this warning
    case RC_INTERNAL_APPLY:
    ^
    __attribute__((fallthrough));
interp.c:831:5: note: insert 'break;' to avoid fall-through
    case RC_INTERNAL_APPLY:
    ^
    break;
1 error generated.

It seems the label should be added with a break or an annotation to
shut up the compiler.