build: Update for install tool usage.

Export install symbol from master makefile.
Use install tool to set header file permissions when installing.

Ensure header files have 0644 permissions

Signed-off-by: Robert Walker <robert.walker@arm.com>
diff --git a/decoder/build/linux/makefile b/decoder/build/linux/makefile
index 15d0da9..330829b 100644
--- a/decoder/build/linux/makefile
+++ b/decoder/build/linux/makefile
@@ -68,7 +68,7 @@
 export MASTER_CPP=$(CROSS_COMPILE)g++
 export MASTER_LINKER=$(CROSS_COMPILE)g++
 export MASTER_LIB=$(CROSS_COMPILE)ar
-INSTALL=install
+export INSTALL=install
 
 # installation directory
 INSTALL_LIB_DIR=/usr/lib/