haproxy: Fix PCRE_DIR include path

haproxy: Fix PCRE_DIR include path

haproxy's Makefile already happens include/ after specifying PCRE_DIR
which would result in the following build errors on an external
toolchain that does not automatically search for headers in
$(STAGING_DIR):

mipsel-linux-gnu-gcc -Iinclude -Iebtree -Wall  -O2 -g
-fno-strict-aliasing -Wdeclaration-after-statement  -DBUFSIZE=16384
-DMAXREWRITE=1030 -DSYSTEM_MAXCONN=165530      -DCONFIG_HAP_LINUX_SPLICE
-DTPROXY -DCONFIG_HAP_LINUX_TPROXY -DCONFIG_HAP_CRYPT -DUSE_ZLIB
-DENABLE_POLL -DENABLE_EPOLL -DUSE_CPU_AFFINITY -DASSUME_SPLICE_WORKS
-DUSE_ACCEPT4 -DNETFILTER -DUSE_GETSOCKNAME -DCONFIG_REGPARM=3
-DUSE_OPENSSL  -DUSE_SYSCALL_FUTEX -DUSE_LUA
-I/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/lua-5.3.3/include
-DUSE_PCRE
-I/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/include/include
-DUSE_PCRE_JIT  -DCONFIG_HAPROXY_VERSION=\"1.7.4-patch02\"
-DCONFIG_HAPROXY_DATE=\"2017/04/01\" \
      -DBUILD_TARGET='"linux2628"' \
      -DBUILD_ARCH='""' \
      -DBUILD_CPU='"generic"' \
      -DBUILD_CC='"mipsel-linux-gnu-gcc"' \
      -DBUILD_CFLAGS='"-O2 -g -fno-strict-aliasing
-Wdeclaration-after-statement -DBUFSIZE=16384 -DMAXREWRITE=1030
-DSYSTEM_MAXCONN=165530"' \
      -DBUILD_OPTIONS='"USE_LINUX_SPLICE=1 USE_LINUX_TPROXY=1
USE_ZLIB=yes USE_REGPARM=1 USE_OPENSSL=1 USE_LUA=1 USE_PCRE=1
USE_PCRE_JIT=1"' \
       -c -o src/haproxy.o src/haproxy.c
In file included from include/types/proxy.h:33:0,
                 from include/proto/log.h:32,
                 from include/common/cfgparse.h:29,
                 from src/haproxy.c:65:
include/common/regex.h:31:18: fatal error: pcre.h: No such file or
directory
 #include <pcre.h>

Fix this by stripping the additional include/ being added.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>