src/Makefile.am: fix building with --enable-gsmhr

In change [1] I moved libraries from _LDFLAGS to _LIBADD, but I did
not notice that there was another problem: when building with gsmhr,
the content of _LIBADD gets overwritten in a conditional block.

Do not overwrite but append libgsmhr.la to _LIBADD.

Change-Id: Ib46b1f6555b13e0937a8d6fe6e2ad70ed9d06d58
Fixes: [1] 08c1edb986cc9ae9cbabe0973cc61309025f4e7f
---
M src/Makefile.am
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/Makefile.am b/src/Makefile.am
index 9cbe492..550e3b8 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -35,7 +35,7 @@
 	$(NULL)
 
 if ENABLE_GSMHR
-libosmogapk_la_LIBADD = $(top_builddir)/libgsmhr/libgsmhr.la
+libosmogapk_la_LIBADD += $(top_builddir)/libgsmhr/libgsmhr.la
 endif
 
 # Processing queue implementation

