When setting the default value, the macro name was specified
as GIT_FLEX_ARRAY, which is inconsistent with it's earlier
usage in the file. This caused a compilation error, using the
MS Visual C/C++ compiler, when compiling the git_packlist
struct definition in src/odb.c.

In addition to changing the spelling of the FLEX_ARRAY macro
to GIT_FLEX_ARRAY, including it's use in src/odb.c, we also
rename the TYPEOF macro to GIT_TYPEOF.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>