loadsvf: Declare JTAG_set_mode in jtag.h

Declare JTAG_set_mode() in jtag.h to prevent an implicit function
declaration compiler error when compiling with GCC 15.

In loadsvf, main.c calls JTAG_set_mode(), which is implemented in
src/jtag.c but was omitted from the header src/jtag.h. Under GCC 15,
-Werror=implicit-function-declaration is enabled by default, causing
the compilation of main.c to fail.

Google-Bug-Id: 568356611
Tested: BitBake built loadsvf successfully
Change-Id: I4140592ad10fb100d715c6d8c1741847f3f6be9e
Signed-off-by: Bart Van Assche <bvanassche@google.com>
diff --git a/meta-gbmc-staging b/meta-gbmc-staging
index 34b83bd..8f3194d 160000
--- a/meta-gbmc-staging
+++ b/meta-gbmc-staging
@@ -1 +1 @@
-Subproject commit 34b83bd39c662d99dd9ad82f12289d133d8ba038
+Subproject commit 8f3194d69dc76b0d06909df114691563a6fa62d9