BaseTools: Fix PLATFORM_DIR variable value.
BaseTools: Fix PLATFORM_DIR variable value.

In commit 017fb1cd4c5e3c8b914eb217ac1760223687dad7, the PLATFORM_DIR
macro has been updated to resolve to the correct path. However, it is
incorrectly accessed via curved rather than curly braces by GenMake.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Marvin Haeuser <698b52c00dda40a38da3fa2eb3df3afebf936977@outlook.com>
Reviewed-by: Yonghong Zhu <78d981fe33a016071af5222c56d5cf77f30fb0b2@intel.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2