bluez: Fix invalid-pp-token warning found with clang

bluez: Fix invalid-pp-token warning found with clang

We add FIRMWARE_DIR via CFLAGS using escapes for string and clang
option parser flags it

<command line>:4:23: warning: missing terminating '"' character [-Winvalid-pp-token]
    4 | #define FIRMWARE_DIR \"/usr/lib/firmware\"
      |                       ^

emitting this warning during configure means some tests fail wrongly. e.g.
test for enable -fPIC -DPIC, which results in textrels in libbbluetooth.so

Pass the string definition using single quotes to avoid the shell
"eating" the quotes and other characters

This works well across gcc and clang

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>