makefile: Fix build from an external path.

When you build from an external path, compiler can't find convert.h
include, because it was specified relative to the current directory.
Change this to specify the include dit relative to the Makefile
location.

Signed-off-by: Tom Tsou <tom.tsou@ettus.com>
diff --git a/Transceiver52M/Makefile.am b/Transceiver52M/Makefile.am
index d0a20f4..4ec06e0 100644
--- a/Transceiver52M/Makefile.am
+++ b/Transceiver52M/Makefile.am
@@ -21,7 +21,7 @@
 
 include $(top_srcdir)/Makefile.common
 
-AM_CPPFLAGS = -Wall $(STD_DEFINES_AND_INCLUDES) -I./common
+AM_CPPFLAGS = -Wall $(STD_DEFINES_AND_INCLUDES) -I${srcdir}/common
 AM_CXXFLAGS = -ldl -lpthread
 
 SUBDIRS = arm x86