compat/unsetenv.c: Fix a sparse warning

The gitunsetenv function includes an (redundant) declaration of the 'environ' symbol, which is a pointer to the table of environment variables. Unfortunately, on MinGW, this provokes sparse to issue the following warning: compat/unsetenv.c:5:20: warning: non-ANSI function declaration of \ function '__p__environ' On MinGW, the <stdlib.h> header defines the 'environ' symbol as a preprocessor macro (via _environ) which obtains the environ table pointer via a call to the __p__environ() function. In order to suppress the warning, we simply remove the redundant declaration of the 'environ' symbol, since the symbol is already declared correctly in <stdlib.h> (included via git-compat-util.h). Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>