[pbc] Correct compilation warnings (#5794)

ports/pbc/CONTROL

@@ -1,4 +1,4 @@
 Source: pbc
-Version: 0.5.14
+Version: 0.5.14-1
 Build-Depends: mpir
 Description: Pairing-Based Crypto library provides low-level routines for pairing-based cryptosystems.

ports/pbc/windows.patch

@@ -1,3 +1,25 @@
+diff --git "a/include/pbc_curve.h" "b/include/pbc_curve.h"
+--- "a/include/pbc_curve.h"
++++ "b/include/pbc_curve.h"
+@@ -60,7 +60,9 @@
+ 
+ void field_curve_set_quotient_cmp(field_ptr c, mpz_t quotient_cmp);
+ 
++#ifdef __GNUC__
+ #pragma GCC visibility push(hidden)
++#endif
+ // Internal:
+ 
+ element_ptr curve_x_coord(element_t e);
+@@ -74,6 +76,8 @@
+ void curve_set_si(element_t R, long int x, long int y);
+ void curve_set_gen_no_cofac(element_ptr a);
+ 
++#ifdef __GNUC__
+ #pragma GCC visibility pop
++#endif
+ 
+ #endif //__PBC_CURVE_H__
 diff --git "a/include/pbc_vc_compat.win32.h" "b/include/pbc_vc_compat.win32.h"
 index 27d3bba..7f772d4 100644
 --- "a/include/pbc_vc_compat.win32.h"