package/bootstrap: fix licence file path

The licence file is in css/ directory.

Fixes:
http://autobuild.buildroot.net/results/0d4/0d4b9488d1aa80e8d290e4dfe1937c3879eceea4

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/bootstrap/bootstrap.hash

@@ -1,3 +1,3 @@
 # Locally computed:
 sha256	fa1caaaf640c32c553d0b323da1036f2232703d64a1d22c3a2ec84ace68c791a	bootstrap-4.0.0-dist.zip
-sha256	8c1a4c25634d5841924aab1848acc9dcbc3d5672183053c8b71ff2139b65d7c3	bootstrap.css
+sha256	8c1a4c25634d5841924aab1848acc9dcbc3d5672183053c8b71ff2139b65d7c3	css/bootstrap.css

package/bootstrap/bootstrap.mk

@@ -8,7 +8,7 @@ BOOTSTRAP_VERSION = 4.0.0
 BOOTSTRAP_SITE = https://github.com/twbs/bootstrap/releases/download/v$(BOOTSTRAP_VERSION)
 BOOTSTRAP_SOURCE = bootstrap-$(BOOTSTRAP_VERSION)-dist.zip
 BOOTSTRAP_LICENSE = MIT
-BOOTSTRAP_LICENSE_FILES = bootstrap.css
+BOOTSTRAP_LICENSE_FILES = css/bootstrap.css
 
 define BOOTSTRAP_EXTRACT_CMDS
 	$(UNZIP) $(DL_DIR)/$(BOOTSTRAP_SOURCE) -d $(@D)