hostapd: escape double quoutes in wpad CFLAGS

hostapd: escape double quoutes in wpad CFLAGS

A recent commit in hostapd added a build option to specify the default
TLS ciphers. This build option is passed via CFLAGS. Due to the way
CFLAGS are handled when building wpad, the compiler tries to recursively
expand TLS_DEFAULT_CIPHERS, resulting in the following error:

../src/crypto/tls_openssl.c: In function 'tls_init':
<command-line>:0:21: error: 'DEFAULT' undeclared (first use in this function)
../src/crypto/tls_openssl.c:1028:13: note: in expansion of macro 'TLS_DEFAULT_CIPHERS'
   ciphers = TLS_DEFAULT_CIPHERS;
             ^

Escape double quotes in the .cflags file to avoid this.

Fixes: 2f78034c3ef ("hostapd: update to version 2017-08-24")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>