thread-utils.h: Avoid using a non-standard C construct

In particular, in standard C, a struct or union must have at
least one member declared (ie. structs and unions cannot be
empty). Some compilers allow empty structs as an extension
and won't even issue a warning unless asked for it (eg, gcc
requires -pedantic). Some compilers allow empty structs as
an extension and will only treat it as an error if asked for
strict checking (eg Digital-Mars with -A). Some compilers
simply treat it as an error (eg MS Visual C/C++).

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Andreas Ericsson <ae@op5.se>

src/thread-utils.h

@@ -69,7 +69,7 @@ GIT_INLINE(int) gitrc_dec(git_refcnt *p)
 # error GIT_THREADS but no git_lck implementation
 
 #else
-typedef struct {} git_lck;
+typedef struct { int dummy; } git_lck;
 # define GIT_MUTEX_INIT   {}
 # define gitlck_init(a)   (void)0
 # define gitlck_lock(a)   (void)0