librtas: avoid bashism in rules.mk
librtas: avoid bashism in rules.mk

Ubuntu has made dash as its default shell (/bin/sh). This leads to build
being broken on ubuntu.
More information at: https://wiki.ubuntu.com/DashAsBinSh
This patch removes bashism and hence fixes the build breakage.

Signed-off-by: Maran Pakkirisamy <ac4df164b43a127d4b7dbe52bbc68af81d6a42c5@linux.vnet.ibm.com>

nfont/librtas,nfont/librtas