video: move extern declarations from C to headers

This moves some extern declaration from lcd.c to lcd.h, removing
unneeded ifdef around a pair of them.  Additionally, since
gunzip_bmp() was declared static in cmd_bmp.c but extern in lcd.c, I
removed the static.  The extra "#include <lcd.h>" in cmd_bmp.c is
added to ensure the header is consistent with the source.

This has been compile-tested on both ARM (at91 boards) and PowerPC
(HH405_config, TQM823L_LCD_config, mcc200_config), to test all use
combinations.

Signed-off-by: Alessandro Rubini <rubini@gnudd.it>
[agust@denx.de: removed gunzip_bmp() fixes as commit c01171ea did it]
Signed-off-by: Anatolij Gustschin <agust@denx.de>