I'm not sure, is it bug or "as intended" but previous versions of stlink (1.6.0 and earlier) installed main include/stlink.h and several other headers under include/stlink/.
1.6.1 installs include/stlink.h, include/stm32.h and several other headers under include/stlink/.
Is it as intended? stm32.h looks too generic to be installed into system-wide include directory, isn't it?