gcc: fix ccache hash of patches in BR2_GLOBAL_PATCH_DIR

gcc: fix ccache hash of patches in BR2_GLOBAL_PATCH_DIR

In commit f4682cf933, a hash of the patches applied to gcc was created
to make sure that ccache can properly detect when the toolchain has
changed. The patches applied to gcc consist of the buildroot patches in
package/gcc, but also potentially patches in BR2_GLOBAL_PATCH_DIR.
However, the path to the patches in BR2_GLOBAL_PATCH_DIR was corrected
incorrectly, because it misses a /. So instead of:

$(BR2_GLOBAL_PATCH_DIR)/gcc-initial/*.patch

it would look for

$(BR2_GLOBAL_PATCH_DIR)gcc-initial/*.patch

In other words, if BR2_GLOBAL_PATCH_DIR doesn't end with /, the patches
in BR2_GLOBAL_PATCH_DIR are not taken into account in the ccache hash.

To fix, add the missing /

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: He Chunhui <hchunhui@mail.ustc.edu.cn>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>