Given the command

flatc -o . -c test.fbs

it would generate header file

.test_generated.h

rather than

./test_generated.h

This fixes this issue.

Tested:
Manually verified that flatc generates the correct output files given output
paths ending with and without '/' on Linux and Windows.

Bug: 16464827
Change-Id: I854cb881286f22690f1885f942cf3fd2fc59ca8d