common-tasks.rst: Fix GNU_HASH in hello.bb

Fix the following GNU_HASH issue in "Single .c File Package
(Hello World!)" example:

ERROR: hello-1.0-r0 do_package_qa: QA Issue: File /usr/bin/helloworld in package hello doesn't have GNU_HASH (didn't pass LDFLAGS?) [ldflags]

Reported-by: Matthew Tsenkov <matthew.tsenkov@konsulko.com>
(From yocto-docs rev: 99a53f443193c0becd48a07a2dd740bb417dcf49)

Signed-off-by: Leon Anavi <leon.anavi@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/common-tasks.rst

@@ -2298,7 +2298,7 @@ directory BitBake uses for the build.
    S = "${WORKDIR}"
 
    do_compile() {
-       ${CC} helloworld.c -o helloworld
+       ${CC} ${LDFLAGS} helloworld.c -o helloworld
    }
 
    do_install() {

documentation/ref-manual/examples/hello-single/hello.bb

@@ -8,7 +8,7 @@ SRC_URI = "file://helloworld.c"
 S = "${WORKDIR}"
 
 do_compile() {
-	${CC} helloworld.c -o helloworld
+	${CC} ${LDFLAGS} helloworld.c -o helloworld
 }
 
 do_install() {