Fix some "implicit declaration of function" warnings
In particular, the compiler issues the following warnings:
cc -g -O2 -Wall -Wextra -I../src -c t0602-write.c -o t0602-write.o
t0602-write.c: In function `testfunc__index_load_test':
t0602-write.c:45: warning: implicit declaration of function \
`git_index__write'
cc -g -O2 -Wall -Wextra -I../src -c t0603-sort.c -o t0603-sort.o
t0603-sort.c: In function `testfunc__index_sort_test':
t0603-sort.c:45: warning: implicit declaration of function \
`git_index__sort'
In order to suppress the warnings, we add appropriate external
declarations of git_index__{write,sort} to the index.h header file.
Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>